Groundwater and Soil Cleanup Improving Management of Persistent Contaminants (1999) / Chapter Skim
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3 Metal and Radionuclides: Technologies for Characterization, Remediation, and Containment
3 Metal and Radionuclides: Technologies for Characterization, Remediation, and Containment
Pages 72-128
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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.
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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.
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74 GROUNDWATER AND SOL CLEANUP TABLE 3-2 Half-Lives of Radioactive Compounds Common at DOE Installations Radionuclide Isotope Half-Life (years) Tc 99 2.12 x 105 U 234 2.47x 105 235 7.1 x 108 238 4.51 x 109 Cs 137 30 Sr 90 28 Pu 238 86 239 24.4 x 103 240 6.58 x 103 3H 12 Th 228 1.91 230 8.0 x 104 232 1.41 x 101° SOURCE: Weast, 1980.
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Elements with Multiple Oxidation States Element Oxidizing Conditions Reducing Conditions Tc Tc(VII) : TcO4-, high solubility, very weak adsorption Tc(IV)
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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.
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79 o ·_1 he ·_1 o o ~ ~ u ~ x cn o o u ¢ v)
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have been measured, provided appropriate stability constants are available (Nordstrom et al., 1979~. Commonly used programs include MINTEQA2 (Allison et al., 1991)
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and Stumm (1992~. The description of metal complexation with natural organic matter (NOM; for example, humic substances)
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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.
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The procedures have been criticized as being unable to provide accurate information about the associations of trace metals (Martin et al., 1987; Kheboian and Bauer, 1987; Rapin et al., 1986; Rendall et al., 1980; Sheppard and Stephenson, 1997; Tipping et al., 1985~. Criticisms have focused mainly on the application of the procedures to the assessment of associations of cationic metals with specific solid phases.
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Because of their location at the surface, caps generally have a more sophisticated layered structure than vertical barriers and can be instrumented much more easily with water collection systems and sensors. Their role in groundwater contaminant transport is limited to reducing leaching from the vadose zone to the groundwater.
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Another hydraulic aspect of vertical barriers is that the presence of the barrier may affect the surrounding groundwater flow. For example, if a site is completely surrounded by a vertical barrier, groundwater will "mound up" at the upgradient edge of the barrier.
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Similarly, installing barriers in aquifers consisting of large cobbles may be difficult. Effects of Contaminant Properties and Site Conditions In addition to hydrologic and geologic conditions, the success of vertical barriers may depend on a number of other processes related to contaminated properties and site conditions.
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Trenching is best suited to large sites in which the contaminant zone can be surrounded while a minimum amount of contaminated soil is excavated and at which complete treatment of the contaminated materials would be prohibitively expensive. Like other physical containment methods, trenched barriers have limited application at depths greater than 30 m (100 ft)
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. More recently, the use of driven sheet pile is increasing in "funnel-and-gate" applications of permeable barriers, in which the sheet pile directs contaminated groundwater into a wall section containing media designed to react with the contaminants.
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Perhaps equally as important, sheet pile can be removed when it is no longer needed. In this context, sheet pile couples well with aggressive in situ treatment technologies, such as chemical flooding systems (discussed later in this chapter)
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SOURCE: File et al., 1996. Bottom Barriers Bottom barriers, emplaced beneath existing in situ contaminants, have a number of features in common with vertical barriers, including the fact that (1)
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However, at this time there is little experience either within DOE or in the private sector in constructing bottom barriers by using directional drilling. Given the trend in recent years toward risk reduction by containment, significant advances likely will be made in bottom barrier installation by directional drilling in the next few years.
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Tracer Tests Tracer tests can pinpoint leaks in barriers, but they require installation of numerous monitoring points around the barrier (Williams et al., 1997~. Using tracer tests for barrier verification has proven very feasible in the vadose zone, where gas-phase tracers can be used and diffusion coefficients are relatively high and isotropic.
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Depending on the hydrogeologic and chemical setting, these sensors may directly indicate the presence of water (e.g., in the vadose zone) , or they may be selective for specific chemical contaminants.
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The broad categories of immobilization technologies discussed in this section include in situ vitrification, solidification and stabilization, permeable reactive barriers, in situ redox manipulation, and bioremediation. In Situ Vitrification Description In situ vitrification (ISV)
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Physical and Chemical Principles The development of in situ vitrification was based on principles of ex situ vitrification processes, which are well established. Soils containing sufficient concentrations of conductive cations are slightly conductive to electricity, so electric currents can pass through.
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. If intact steel drums containing organic liquids are present, pretreatment is required to rupture the drums.
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Pretreatment is required for intact steel drums containing organic liquids. ISV can tolerate waste and debris within the treatment zone.
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under the following conditions: · depth greater than about 6 m (20 ft) ; · excessive moisture levels or high moisture recharge rates, especially if volatile organic compounds are present; · presence of operational utility trenches within the treatment zone; · presence of intact steel drums containing organic liquids; · a matrix composition that results in an excessively high melting temperature or that will not form a glass and/or crystalline product upon melting and cooling;
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Ex situ processes are among the most mature technologies, and excavated soils are frequently treated prior to disposal. Solidification and stabilization procedures have been described by Smith et al.
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. Projects have included construction of a 20-m-deep soil-bentoite wall to contain groundwater contamination in a former waste pond and shallow soil mixing and stabilization of 82,000 yd3 of contaminated soil at a former manufactured gas plant site.
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. Permeable Reactive Barriers Description A permeable reactive barrier is a passive in situ treatment zone of reactive material that immobilizes metal or radionuclide contaminants as groundwater flows through it (Vidic and Pohland, 1996; EPA, 1998; Schultz and Landis, 1998~.
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A third approach is to install temporary sealable sheet piling to allow dewatering and installation of a reactive zone. A fourth approach is to inject the reactive material directly with a jet.
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Performance Table 3-10 provides examples of permeable reactive barrier installations with various reactive media. One of the most carefully studied reactive barriers is at Elizabeth City, North Carolina.
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Limitations Because metals and radionuclides are nondegradable, treatment by sorption or precipitation within a reactive barrier must be regarded as a retardation of contaminant migration rather than as a permanent solution to the problem. If retardation is accomplished through reduction to an immobile form, either the reducing conditions must be maintained to prevent remobilization or the reduction reaction must be effectively irreversible.
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This type of technology can be viewed as a special type of permeable reactive barrier, in which a part of the subsurface is transformed to a containment treatment zone.
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Physical and Chemical Principles The primary redox buffer in the reducing zone created by in situ redox manipulation systems is generally structural iron that is, iron bound in clay interlayers of the aquifer material. This reduced iron forms a large reducing buffer region that is not reoxidized easily by oxygen, yet reacts readily enough with the contaminants to retard their transport.
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From 60 to 100 percent of the iron in the sediments was reduced by dithionite. No significant plugging of the aquifer formation (a potential problem TABLE 3-11 Summary of In Situ Redox Manipulation Tests Reactive Medium Contaminants Study Type Site Reference Dithionite Cr Field injection to reduce structural Hanford 100-Hf full scale PNNL, 1996; Fruchter et al., 1997 iron Field Hanford 100-Hf Vermeul et al., 1995; push-pulla Fruchter et al., 1996 Intermediate Physical model of 7-m-radius; 10-degree wedge of contaminated aquifer Fruchter et al., 1996 a A push-pull test uses a single well for both injection of reactive agents and withdrawal of water samples
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Because of the success of this field test, a large-scale demonstration of in situ redox manipulation is now under way at Hanford. In this demonstration, a treatment zone approximately 50 m (150 It)
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If reducing conditions can be maintained by the addition of substrate and suitable nutrients, inorganic contaminants will remain in their highly insoluble, immobile forms. Application No definitive field studies have been reported specifically on manipulating the subsurface environment to cause microbiological reduction and immobilization of metals and radionuclides.
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TECHNOLOGIES FOR MOBILIZING AND EXTRACTING METALS AND RADIONUCLIDES In addition to being treated by immobilization, some metals and radionuclides in groundwater and soil can be mobilized and extracted from the subsurface for treatment or disposal at the surface. Electrokinetic, soil flushing, soil washing, and phytoremediation processes, discussed below, are the primary technologies being developed for this purpose.
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Application Electrokinetic processes have been used for the remediation of soils containing a number of inorganic contaminants (EPA 1997b,c)
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Advantages Electrokinetic systems can mobilize both metals and organic compounds as a result of the several processes that are responsible for contaminant movement. Description Soil Flushing and Washing Soil flushing is an in situ process and soil washing is an ex situ process in which contaminants are removed from the soil by using a suitable extracting solution.
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For soil flushing, extractant chemicals are applied to the contaminated soil by surface flooding, sprinklers, leach fields, vertical or horizontal injection wells, basin infiltration systems, or trench infiltration systems (Evanko and Dzombak, 1998~. After contact with the contaminated soil, the extractant is recovered for disposal or treatment and reuse.
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Soil flushing has the added advantage of requiring no excavation. Phytoremediation Description Phytoremediation refers to the use of plants to extract metals and metalloids from contaminated soils as shown in Figure 3-8.
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· Rhizodegradation involves stimulating microorganisms around the root zone, resulting in enhanced microbial degradation of the contaminants. · Phytodegradation refers to the transformation of organic contaminants to less toxic compounds through their adsorption, uptake, or degradation by either the plant itself or plant-associated microflora.
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. 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.
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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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· 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.
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Pp. 1053-1059 in Proceedings of the International Containment Technology Conference, St.
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Pp. 607-613 in Proceedings of the International Containment Technology Conference, St.
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Pp. 704-710 in Proceedings, International Containment Technology Conference, February 9-12.
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1996. Vertical barriers: Sheet piles.
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1999. Permeable Reactive Barrier Installation Profiles.
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Pp. 881-887 in Proceedings of the International Containment Technology Conference, St.
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Pp. 1039-1045 in Proceedings of the International Containment Technology Conference, St.
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