Technical and Policy Challenges in Deep Vadose Zone Remediation

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Review: Technical and Policy Challenges in Deep Vadose Zone Remediation of Metals and Radionuclides P. Evan Dresel,*,†,‡ Dawn M. Wellman,† Kirk J. Cantrell,† and Michael J. Truex† † ‡

Pacific Northwest National Laboratory, Richland, Washington 99352, United States Department of Primary Industries, Bendigo, Victoria 3554, Australia ABSTRACT: Contamination in deep vadose zone environments is isolated from exposure so direct contact is not a factor in its risk to human health and the environment. Instead, movement of contamination to the groundwater creates the potential for exposure and risk to receptors. Limiting flux from contaminated vadose zone is key for protection of groundwater resources, thus the deep vadose zone is not necessarily considered a resource requiring restoration. Contaminant discharge to the groundwater must be maintained low enough by natural attenuation (e.g., adsorption processes or radioactive decay) or through remedial actions (e.g., contaminant mass reduction or mobility reduction) to meet the groundwater concentration goals. This paper reviews the major processes for deep vadose zone metal and radionuclide remediation that form the practical constraints on remedial actions. Remediation of metal and radionuclide contamination in the deep vadose zone is complicated by heterogeneous contaminant distribution and the saturation-dependent preferential flow in heterogeneous sediments. Thus, efforts to remove contaminants have generally been unsuccessful although partial removal may reduce downward flux. Contaminant mobility may be reduced through abiotic and biotic reactions or through physical encapsulation. Hydraulic controls may limit aqueous transport. Delivering amendments to the contaminated zone and verifying performance are challenges for remediation.

’ INTRODUCTION Deep vadose zone contamination is a significant issue in all regions of the U.S. although much of the focus has been on arid and semiarid regions.1 For the purpose of this paper we define the deep vadose zone as the sediments or rock below the zone of practicable excavation and removal but above the water table. Implicit in this definition is an assumption that there is no risk from direct exposure to deep vadose zone contamination. In situ remedies are potentially viable to reduce contaminant toxicity and/or mobility so remediation of the deep vadose zone provides the opportunity to keep contamination from migrating beyond the source area. Although the lower boundary of the deep vadose zone is well-defined, the upper boundary depends on site-specific factors and the regulatory decision process. This review will use the generic term “sediment” for unsaturated zone lithologies. The deep vadose zone is a concern from the perspective of environmental protection because it represents a potential source of ongoing contamination to groundwater and associated receptors. Contaminant transport mechanisms through the vadose zone can attenuate the overall contaminant flux to the groundwater, and vadose zone contamination may not necessarily require remediation if the natural flux results in sufficiently low contaminant concentrations in the groundwater. Remediation to control transport, enhance attenuation mechanisms, or remove contaminant may be needed to limit flux so groundwater or surface water protection standards are maintained. r 2011 American Chemical Society

Technical challenges complicate the decision process for deep vadose zone remedial actions. Remediation options for the deep vadose zone are less developed than the options for shallow soil contamination or for saturated groundwater contamination. In spite of active research and development, few remediation technologies have been tested in the field, and fewer have been successfully implemented as full remedial actions. It is often difficult to determine if reported test results are overly optimistic for successful completion of remediation; as such, important issues of economics and applicable scale factors are not specifically addressed. This manuscript presents a review of how hydrogeologic, geochemical, and biogeochemical processes may operate in the deep vadose zone and be used to meet remedial objectives, the technical risks and challenges for consideration during evaluation of a proposed remedial action, additional development needs for deep vadose zone remediation techniques, and the benefits of applying vadose zone remediation for groundwater protection. Specific examples are presented to highlight particular considerations and further the conceptual understanding of key concepts, especially those relevant to Cr(VI), uranium, Received: April 15, 2010 Accepted: February 11, 2011 Revised: February 8, 2011 Published: March 11, 2011 4207

dx.doi.org/10.1021/es101211t | Environ. Sci. Technol. 2011, 45, 4207–4216

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Figure 1. a) Conceptual diagram of vadose zone fluxes after waste discharge and moisture redistribution and b) postremediation fluxes.

technetium-99, and mercury contamination. These contaminants are important long-term risk drivers at government and commercial sites; however, the concepts are generally extensible to other metals.

’ VADOSE ZONE CONCEPTUAL MODEL AND REMEDIATION APPROACH Aqueous waste discharged at the ground surface and allowed to move through the deep vadose zone, often under unsaturated flow conditions, is acted on by capillary and gravitational forces. Figure 1 is a conceptual diagram of a contaminated vadose zone depicting the contaminant conditions that are the target of deep vadose zone remediation for this review. Under unsaturated conditions water is held under tension, and capillary force causes moisture accumulation to occur preferentially in finer sediments. When coarse sediments are encountered during downward flow, the capillary change causes a capillary break, leading to lateral spreading and accumulation in finer-grained layers until the water saturation and associated pressure increases sufficiently to allow vertical flow into the coarser sediments. Numerous texts on unsaturated flow are available, and the topic has been covered in detail with respect to vadose zone contamination.2 After waste discharge ceases, continued water movement through the vadose zone is a function of the moisture distribution, typically elevated compared to unimpacted conditions, the net rate of water infiltration at the surface (i.e., water that moves beyond the zone of evapotranspiration), and the hydraulic conductivity of the sediments. The moisture distribution at the time of remediation depends on the history of waste discharge and water redistribution, and on the distribution of finer and coarser grained layers or lenses. Fully saturated flow and formation of perched aquifers may occur during waste discharge or due to water infiltration. The physical processes of dispersion and diffusion, both on a pore water and intragranular scale, can also impact pore water contaminant concentration and associated flux to the groundwater. Contaminant interactions with the sediment, such as sorption, precipitation, or dissolution reactions are important to consider as attenuation mechanisms that can limit the flux of contaminants to the groundwater. In contrast to groundwater conditions, the presence of a gas-phase in the unsaturated

vadose zone also needs to be considered with respect to pore water chemistry for both natural and remediation processes.3,4 For instance, under natural conditions, oxygen and carbon dioxide in the gas phase may have important impacts on pore water chemistry, cause precipitation or dissolution of mineral phases, and affect biological activity. Biologic activity may lead to anoxic conditions and/or increase carbon dioxide partial pressure. The conditions in the deep vadose zone have both positive and negative impacts on remediation of contaminants. Positive aspects with respect to remediation include the following: • unsaturated flow is often slow and incremental enhancements may meet remediation goals • due to slow movement, long times may be available for interactions with sediments and remediation amendments • finer-grained units usually have higher water-holding/sorption capacity and hence hold more contaminants • a large portion of the driving force for water flow comes from meteoric recharge and can therefore be controlled with surface caps or infiltration barriers • gas-phase advection for distribution of remediation amendments can be effective over large zones of influence without introducing large quantities of water • if water is added in relatively small quantities, its effect on the rate of contaminant movement in the deep vadose zone may still be low. Challenges to remediation may include the following: • heterogeneity in hydraulic properties, as in the saturated zone, may cause difficulties for bringing remediation amendments in contact with contaminants or for extracting contaminants o preferential liquid flow paths can vary greatly under different saturations o gas-phase movement is preferential in higher permeability zones and higher liquid saturation can reduce gas permeability o contaminants held in low permeability finer-grained materials may be harder to access for contaminant removal or treatment • introduced water may have undesirable effects on moisture distribution and unsaturated flow o increased flow is often counter to the goal of reducing downward flux 4208

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Environmental Science & Technology o contaminants can be mobilized ahead of added remediation solutions rather than mixing with these solutions and being treated • remediation chemistry can be affected by the presence of gas phase (e.g., CO2, oxygen) and low soil-moisture content o reoxidation can limit the effectiveness of reductive treatments • difficulty of characterizing large-scale hydraulic properties due to inability to stress significant volumes of sediment • monitoring the pore water for characterization/remediation can be difficult because deep pore water cannot be readily collected and measurements may represent only a small area around the collection point. Within the context of the deep vadose zone conditions, the conceptual approaches for remediation can be categorized as controlling the flux of contaminants to groundwater to meet groundwater remediation goals either through 1) contaminant mass reduction, 2) stabilization of contaminants in less mobile forms, or 3) reduction of the movement of contaminated pore water. These contaminant flux-reduction based approaches recognize that for the deep vadose zone full removal of contamination is likely technically and/or economically impractical but can still be protective of health and the environment.5,6 The need to understand and predict contaminant transport behavior over longer time scales is illustrated by estimates that more than 100 US Department of Energy sites will have remaining residual contamination at the completion of cleanup programs.7 While there has been much recent research on measuring, estimating, and predicting mass flux, the vast majority is focused on the saturated zone, with minimal attention to characterizing mass flux for vadose zone systems. It is important to note that conceptual models are dynamic, becoming more refined as information about the specific site emerges. Uncertainties in the conceptual model can impact the success of remediation. The extent of contamination and scale of required treatment needs to be considered throughout the research and development process so the remedial alternatives technology targets real-world problems. Some technologies may be viable for high concentration, spatially discrete zones, while others may be more suitable for disseminated contamination.

’ IMPLEMENTATION TECHNIQUES Remedial action must affect the appropriate zone in the subsurface, and the characteristics of the deep vadose zone, discussed above, add considerable complexity compared to groundwater or near-surface remediation. Effective methods for delivery of reagents are among the greatest challenges for in situ deep vadose remediation.8-10 However, unsaturated conditions provide opportunities for introducing and controlling reactants through manipulation of the % saturation or by using gas or multiphase materials. The extent of the treatment zone for liquid additions is dependent on the location and rate of addition and may not reach the contaminated horizons due to heterogeneities and preferential flow paths. Liquid reactant additions may mobilize contaminants along the infiltration front prior to reaction and stabilization, thus driving contaminants toward the water table rather than sequestering them in place.11-13 Two-dimensional experiments have shown alternating rapid and slow infiltration

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can improve uniformity of infiltration in heterogeneous sediments.8 Gas phase reactants or nutrients are attractive for unsaturated sediments in that the gravitational effects on flow are minimal, and the gas permeability of the subsurface is often relatively high.9 However, the gas phase may still be subject to lithologically controlled preferential flow, bypassing contaminated zones. Szecsody et al. recently published a comparison of uranium remediation mechanisms including a number of gaseous treatments.14 Distribution of solid remediation materials in the vadose zone also presents remedial challenges. Slurry emplaced by injection may be affected by preferential flow. Soil mixing has been used to introduce solids or slurry using augers up to ∼4 m diameter with depths reported to 35 m,15 but this will become difficult with increasing depth and areal extent of contamination. The effectiveness of this method is also reduced by indurated sediments or sediments with large cobbles or boulders. Nanoparticles, such as Fe(0) have been considered for vadose application and used for remediation below the water table.16 Nanoparticles can be injected with liquids and thus subject to many of the issues of liquid emplacement. Injection to the saturated zone has been performed as liquid aerosol in an inert gas stream, and this method would work similarly in the vadose zone16 but it may be difficult to produce an even distribution of particles through the treatment zone. Pneumatic fracturing using the gas/aerosol can aid in particle emplacement.16 Foam-based amendment delivery shows promise for in situ remediation. The transport of foam is less subject to gravitational effects, increasing the uniformity of distribution and reducing (but not eliminating) the effects of heterogeneities.11,13,14 Foambased remediation is largely untested. Issues to be considered include limitations on the mass of reactant that can be carried as foam, the pressure required to drive foam flow, and the increase in liquid saturation that will occur at the periphery of a foam injection where foam integrity breaks down, possibly producing undesired gravitational transport. Conceptually foam may include gaseous and liquid reactants. Surfactants in the foam may aid emplacement of nanosized solids, although this is conjecture.

’ REMEDIATION OPTIONS Potential remediation processes for the deep vadose zone that are consistent with the conceptual approach of controlling the contaminant flux to groundwater are discussed below. These vadose zone remediation processes may be broadly categorized as mass reduction, geochemical, biogeochemical, or physical stabilization, and hydraulic control approaches. Mass Reduction. The contaminant mass flux and longevity of that flux for a source zone is a function of both source mass and contaminant transport to the underlying groundwater plume. Complete removal of contaminants dispersed in the deep vadose zone is likely unattainable because of the complexities associated with the distribution and the heterogeneity of subsurface environments.5,6 Where a residual mass of contaminant remains after remediation, it will control mass flux over the long-term. Partial removal of mass, especially in high-concentration zones, if these can be identified and are of limited volume, may be possible and lead to an overall reduction in contaminant flux to the groundwater. However, where the contaminant flux is controlled by desorption or dissolution reactions into the mobile water phase, mass removal may have little effect on the flux. 4209


Environmental Science & Technology Contaminant mass removal must either be achieved by migrating the contaminants through the unsaturated sediment to a point of collection within the vadose zone or by applying sufficient water/chemical to drive contaminants to the groundwater for collection. Vadose Zone Collection. Electrokinetic remediation through application of a low-level direct current often targets lowpermeability sediments where introducing liquid or gas is not practical. Positively charged ions migrate toward the cathode and negatively charged ions to toward the anode. Mobilization of ions occurs through electromigration; water and solute flow toward the cathode is induced through electroosmosis so water additions are needed at the anode for vadose zone remediation.17 Colloids and contaminant-surfactant micelles are transported through electrophoresis.18 Complexant or surfactant addition can enhance mobilization through desorption and micelle formation.19-21 Water is electrolyzed at the anode, producing hydrogen ions and decreasing the pH, while hydroxide ions are produced at the cathode and increase the pH. A number of reviews of the principles and technical challenges of electrokinetic remediation17,22-26 and the applicability to DOE and other radionuclide sites27-29 have been published. Scale-up from laboratory to field conditions has been problematic.30 If contaminants moved by electrokinetic forces are not fully recovered, the remaining contaminants may have been concentrated or moved to areas with lower overall attenuation of contaminant movement. Remaining mobilization chemicals and induced pore water chemistry changes may increase downward migration of residual contaminants. Although active R&D continues, particularly outside the US,31,32 the technical challenges, including placement of the electrodes and control of the electrical potential, controlling undesired reactions in the subsurface, and maintenance of permeability have limited application.30,33 Soil Flushing. Soil flushing can be applied to purposely accelerate movement of contaminants from the vadose zone to the groundwater where the contaminants can then be captured.34 Flushing may be enhanced through addition of extraction solutions such as acids, chemical complexants, addition of competing ions to promote desorption, or manipulation of oxidationreduction state.35,36 Biosurfactants, biosurfactant foam, and exopolysaccharides have been proposed for mobilization of metals in soil, but there has been limited development.37-39 Potential extractants have often been developed for other applications such as in situ mining, heap leaching, and ex-situ soil washing, and it is necessary to evaluate possible negative impacts to the environment such as mobilization of nontarget metals.36,37,40-45 Implementation of soil flushing must consider the potential for lateral spreading and possible bypass of the contaminated zone through preferential vertical flowpaths. Soil flushing is generally working against the dominant processes in the vadose zone that act to retain contaminants in finer-grained materials and maintain relatively slow water and contaminant migration. The ability to control and recover the injected fluids is key to the viability of soil flushing. Recovery of flushing liquids generally means recharge to groundwater followed by groundwater extraction, and there is potential for sustained release to groundwater from residual soil moisture. Soil flushing may be considered where groundwater remediation is already taking place or where groundwater capture can be implemented. Geochemical Stabilization. Metal and radionuclide contaminant interaction with sediments can occur through mineral

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precipitation or surface complexation (sorption) reactions. Manipulation of these reactions can sequester the contaminants above the water table. The valence state frequently has a large influence on sorption and precipitation reactions. Chromium and technetium are highly mobile/poorly sorbed as anionic Cr(VI) chromate and Tc(VII) pertechnetate but strongly sorbed upon reduction. The presence of aqueous complexes may have a large influence on the mobility of contaminants. The prevalent U(VI)-carbonate complexes enhance mobility, while reduction to U(IV) leads to sorption or precipitation. Mineral Precipitation. Mineral precipitation to control contaminant mobility may include precipitation of pure end-members or coprecipitation into solid solutions. The formation of solid solutions can substantially reduce the solubility of the precipitated phase. Mineral precipitation may also physically sequester contaminants by forming overgrowths, thus preventing mobilization of sorbed contaminants and inhibiting reoxidation of contaminants sequestered through reduction processes, discussed below. Phosphate amendments have been investigated for stabilization of lead,46,47 uranium,48-54 mercury,55,56 and strontium90.8,57 Solid phosphate phases are attractive because they are frequently stable under a range of conditions. Soluble phosphates are available for injection or infiltration; however, precipitation reactions may take place so rapidly that formation clogging in proximity to the injection point may occur. Polyphosphate injection can be used to slow the precipitation rate and produce better distribution of the reactants.53 Vadose zone calcite (CaCO3), when present, will control the PCO2, pH, and calcium concentration. Increased PCO2/lower pH causes carbonate mineral dissolution with subsequent decrease in PCO2 and neutralization causing reprecipitation. Strontium-90 and Cr(VI) will substitute into the calcite or aragonite structure.58,59 There has been disagreement on the long-term stability of uranium-substituted calcite for U(IV) and U(VI)60-68 with the more recent studies suggesting high stability or only slow release of uranium from the minerals. Much is known about the stability and reaction rates of carbonate minerals and manipulation of the reactions is conceptually simple, but research is needed to develop practical field-scale systems. Contaminant sequestration in silicate phases is also possible. An increase in pH through addition of ammonia gas or sodium hydroxide liquid or mist will dissolve silicate minerals; subsequent neutralization can incorporate contaminants into reprecipitated silicates.14,69 The silicate minerals are stable or only weather slowly under ambient conditions. Other, more exotic phases that have been suggested for sequestration of contaminants include sulfides for technetium70-72 and the dipotassium salt of 1,3-benzendiamidoethanethiol for removal of remove Hg(II).73 Barium chromate is a moderately low-solubility Cr(VI) solid and forms a solid solution with barium sulfate greatly reducing the solubility when sufficient barium and sulfate are present.74 When considering development of novel sequestrants, the cost and toxicity of reactants, ease of delivery, and ultimate stability need to be assessed. Oxidation/Reduction. Oxidation-reduction processes using chemical reactants can induce contaminant sequestration through formation of low-solubility minerals or increasing sorption to mineral surfaces. Reduction is most commonly proposed for metal and radionuclide contaminants. Many chemical reductants have been considered for in situ remediation of metals and 4210


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Figure 2. Reactions for reduction of metal and radionuclide contaminants (left side of oxidant reactions) and oxidation of potential reductants (left side of reductant reactions). Nitrate is included as a common cocontaminant.

radionuclides including Fe(0), ferrous salts, sodium dithionite, sulfide salts, hydrogen sulfide, and hydroxylamine.75-77 Figure 2 shows reduction reactions for some metals and radionuclides of interest and potential reductants for remediation. The order of the reactions shown is from more oxidizing to more reducing, but kinetic and thermodynamic factors both influence the degree of reaction. H2S has been field tested as an in situ gaseous reductant for chromium78-82 and laboratory tested for uranium.83 SO2 is another gaseous reductant.14,84 The toxicity of H2S and other reductants is a consideration but can be addressed through injection of trace concentrations, monitoring, and engineering controls. Sulfur compounds such as sodium dithionate and calcium polysulfide have been deployed for in situ remediation of Cr(VI) in saturated groundwater75,85-88 and have been investigated for remediation of uranium.89 The dithionate solution reduces Fe(III) in the sediments, and the Fe(II) then acts as a reductant for the contaminant metals. Calcium polysulfide liquid and solid have received considerable attention as a reductant for contaminant metals in shallow soils and the vadose zone.90-93 Granular Fe(0) permeable reactive barriers have been deployed in groundwater for a variety of contaminants including Cr(VI)94-97 and have been investigated for uranium, technetium, mercury, and other metals.98-104 Emplacement of nanoscale particles may offer significant advantages in dispersing the iron through the sediments.105 However, much study is needed to understand the vadose transport of nanoparticles in liquid, aerosols, or as foam at various water saturations. The majority of in situ reductive methods have been applied to the saturated zone or shallow vadose zone. Deoxygenation and some moisture addition may be needed for reaction. Establishing reducing conditions may require high reductant loadings, displacement of air by inert gas, or enhancing microbial biomass (see below). Reoxidation is a serious issue for some contaminants and more important than finding new reductants. Studies indicate that chromium is less subject to reoxidation than technetium or uranium.99,106,107 Once reduced, reoxidation

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may be limited by minimizing water content or physical sequestration through encapsulation or mineral overgrowths. Biogeochemical Stabilization. Manipulation of biological processes can drive or catalyze sequestration reactions. Subsurface microbes are ubiquitous so enhancement of the growth of existing communities may be effective for remediation. Substrates used for enhanced bioremediation of inorganic compounds generally are inexpensive and benign (e.g., vegetable oil, sugars, organic acids). Gaseous substrates need evaluation for deep vadose zone applications. Geochemical conditions including the pH, buffering capacity, oxidation-reduction potential, and the presence of dissolved or solid-phase electron acceptors impact microbial activity. Low moisture conditions in the vadose zone may inhibit the desired biological activity. Thus, ensuring suitable chemical conditions and, in arid regions, sufficient soil moisture will be critical components of vadose zone bioremediation. However, the large solid surface area to water volume suggests that even relatively low-mass biofilms may be effective for remediation in the vadose zone. The majority of bioremediation schemes proposed for metals and radionuclides involve reduction of the contaminant to less mobile and/or less toxic forms. A wide variety of electron acceptors may be used during anaerobic bioremediation and different types of anaerobic bacteria compete for the substrate.108 Direct microbial reduction of chromate,109-111 uranium,109,112 and technetium109,113-116 is distributed among a wide variety of bacteria. Reduction of uranium and technetium is inhibited by denitrification of nitrate116 and other inhibitors such as copper may be important. While reduced chromium is stable, uranium reoxidation can occur with oxygen, nitrate, manganese, and Fe(III) and may be facilitated by microbial activity and humic substances.112,117 McBeth et al.118 found that reduced technetium in aquifer sediments was reoxidized by dissolved oxygen but not nitrate. Thus, as with chemical reduction processes, reoxidation potential will need to be considered in bioremediation applications for the vadose zone. Biological treatment for mercury in wastewater involves bioreduction of Hg(II) to Hg(0) or production of insoluble mercury-sulfur compounds119,120 and possibly could be extended to in situ treatment for ionic mercury contamination. However, unwanted production of methylmercury compounds is an issue,109 and the relative ease of mercury biotransformations make this a less attractive option. Microbial reduction of noncontaminant chemical species may also form precipitates useful for contaminant sequestration. In particular, combinations of reduced iron and sulfur species may cause contaminant coprecipitation or coat sorbed contaminants. Biologically catalyzed precipitation of uranium phosphates is another potential indirect biological sequestration process.121 Biological activity in the subsurface increases biomass, including live and dead bacteria and extracellular organic material that can sorb contaminants.109 The increased biomass may then promote anaerobic conditions and further sequestration reactions. Biomass-related sorption may be a transient phenomenon because of the eventual decay of the biomass but may still provide benefit in reducing contaminant flux to the groundwater. Physical Stabilization. Physical stabilization changes the physical form of the subsurface to trap contaminants mechanically. Injection grouting122 and soil vitrification2,26,123-125 are stabilization techniques that have been applied for shallow wastes. Nominally, these techniques can be extended to the deep vadose zone using the same or similar technical approaches as for 4211


Environmental Science & Technology shallow applications. Stabilization in a grout or glass matrix typically provides considerable hydrologic isolation from the mobile pore water as well as a solid form with lower leachability. For the deep vadose zone, cost and infrastructure requirements may become impractical or prohibitively costly as depth and contaminated volume increase. In situ vitrification or grouting thus is probably best suited to discrete areas of high risk contaminants. Issues regarding steam generation causing release of melted material occurred in early shallow vitrification tests but were addressed by changing to a bottom-up heating method2 and greater overburden for a deep vadose application will probably increase the safety factor. Verification of performance, such as the uniformity of grouting/vitrification also increases in difficulty with depth and volume increase. Thermal treatment may also immobilize contaminants without vitrifying the soil and be more cost-effective than vitrification. For instance, diffusion into crystal matrices at submelting temperature sequestered radionuclides in laboratory experiments.126 Hydrologic Control. Control of moisture movement in the vadose zone can be used to limit the mobility of dissolved or soluble contaminants. This control can take the form of limiting infiltration at the surface, removal of soil-water (desiccation), or reducing the permeability of the sediments. Surface Barriers. Surface barriers or caps are a well established technology for reducing meteoric recharge. Surface barriers may provide significant isolation, but their effectiveness for deep vadose contamination has not been demonstrated and is subject to site-specific factors.127 Caps for very large areas are more difficult to design for runoff control, leachate collection, and physical integrity. Recharge from outside the cover due to lateral flow may be caused by lithologic changes, dipping beds, and small-scale anisotropy in sediments (Figure 1). Subsurface permeability control in the form of slurry walls or grout curtains may then be needed to limit mobilization of contaminants under a cap.128 Long-term maintenance of surface barriers requires a commitment to ongoing care and usually precludes redevelopment on the land. Soil Desiccation. Soil desiccation removes water and limits aqueous transport by increasing the water tension and thus decreasing the unsaturated hydraulic conductivity.129,130 In addition a) the low moisture content of the desiccated zone may act as a capillary barrier to impede flow, b) the soil moisture deficit increases water storage capacity to mitigate the effects of extreme recharge events, and c) desiccation may induce precipitation of mineral phases, sequestering contaminants. Desiccation is temporary in that re-equilibration of moisture will occur over time in the vadose zone, where the rate of rewetting depends on subsurface conditions and the surface recharge rate. However, if moisture conditions had been elevated due to past waste disposal, removal of water along with infiltration control can still render an overall decrease in contaminant movement to the groundwater.130 While simple in concept, field implementation of desiccation for effective moisture control may offer challenges. A field test at the DOE Hanford site is currently underway to investigate the desiccation process.10 Permeability Control. Permeability control limits transport by reducing the interconnected porosity through addition of grout or polymer material or through precipitation of mineral phases. Although there is overlap with physical stabilization, permeability control is primarily concerned with reducing water movement rather than isolation of the contaminant. Permeation grouting has been applied as a subsurface barrier,131 and multiple grouting

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materials may offer possibilities.132-134 Geochemical manipulation to precipitate phosphate, carbonate, or other phases may also have a significant effect on permeability.135,136 Complete filling of the pore space is not necessarily needed if the interconnection between pores can be reduced. Electrokinetic methods have been used to indurate subsurface materials for geotechnical applications and to develop low permeability ironrich bands.137,138 Permeability control using these types of subsurface barriers have the potential to provide more targeted sequestration, lower maintenance, and greater longevity than surface barriers. However, evaluating their performance needs to consider their effectiveness as a function of overall permeability reduction, areal extent of the barrier, and integrity of the barrier, all of which can be difficult to evaluate in the subsurface.

’ KEY REMEDIATION DEVELOPMENT CONSIDERATIONS The prospect for success of any deep vadose zone remediation technology needs to be considered within the regulatory context. When considering remediation feasibility, it is useful to consider the environmental risks and challenges with leaving contaminants in place as part of a flux-control remedy in comparison with the risks associated with contaminant removal and final disposition elsewhere. While contaminant removal is preferred by most regulators and stakeholders, its practical feasibility must be weighed against the strengths and weaknesses of other remedial strategies. Remediation of the vadose zone is typically linked to meeting remediation goals for groundwater. There is a regulatory basis for leaving contamination in place in the vadose zone, provided flux to the groundwater is limited such that groundwater goals are met. As such, remediation in the vadose zone can be targeted to managing the source of contamination and reducing transport through the vadose zone in contrast to meeting a specific concentration measured at some location within the vadose zone. Determining the acceptable level of residual to meet the goals for groundwater receptors, how to appropriately measure when this residual has been obtained, and to monitor over a sustained period are key challenges for setting vadose zone remediation objectives. The objectives must be realistic while realizing that there are considerable uncertainties associated with estimating and measuring contaminant transport at the field scale. Methods and protocols to predict in situ remedial performance are needed to support a remedy decision. While this effort may seem daunting, acceptance of Monitored Natural Attenuation as a remedy for a wide range of contaminants and site settings provides a good example of how predictions of long-term performance can be structured and used to support remedy selection. Techniques suitable for monitoring and managing the long-term presence of contaminants in the vadose zone would be a key component of implementation for a flux-control remedy for the vadose zone. It is relatively straightforward to measure contaminant concentrations in groundwater but more challenging to demonstrate and provide confirmatory monitoring showing that groundwater will remain uncontaminated or that contamination will remain below levels of concern in the future. At the current time, vadose zone remediation approaches for metal and radionuclide contamination based on reduction of the contaminant flux to groundwater are still largely in the developmental and demonstration stage, although there are some 4212


Environmental Science & Technology potentially viable options as described in this review. Transformation of basic science principles into viable deployed systems and transfer of remediation technologies from groundwater, shallow vadose zone, or waste treatment applications to the deep vadose zone generally will require adaptation and demonstration. Remediation techniques that enhance existing attenuation of contaminant transport or further decrease vertical pore water flow by taking advantage of unsaturated flow characteristics seem most likely to have long-term success in mitigating contaminant flux to the groundwater. Effective delivery of remedial amendments to targeted areas of the deep vadose zone is a critical component of development for many of the remediation options and should favor techniques, such as gas-phase advection, that work with the advantageous transport processes in the vadose zone. Breakthroughs in treatment of the deep vadose zone may require a remedial strategy consisting of multiple techniques to overcome inherent shortcomings in individual methodologies. Development efforts for remediation techniques should target collecting sufficient information to enable consideration in a feasibility study. This target generally requires that information from field demonstrations or similar activities is available for evaluating implementation and cost. A balanced long-term approach to support development of fundamental principles and applied technology from different stages of maturity is needed.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ61 3 5430 4425. Fax: þ61 3 5430 4304. E-mail: evan. [email protected]. Corresponding author current address: Future Farming Systems Research Division, Department of Primary Industries, P.O. Box 3100, Bendigo Delivery Centre, VIC 3554, Australia.

’ ACKNOWLEDGMENT This research was supported by the U.S. Department of Energy (DOE) Environmental Management EM-32, Office of Groundwater and Soil Remediation. Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle Memorial Institute. The authors would like to especially thank Mike Perkins of Pacific Northwest National Laboratory for his process conceptualizations and production of the graphics contained within this manuscript. The paper has benefited greatly from the peer review comments. ’ REFERENCES (1) Seaman, J. C.; Looney, B. B.; Harris, M. K. Research in Support of Remediation Activities at the Savannah River Site. Vadose Zone J. 2007, 6 (2), 316–326. (2) Looney, B. B.; Falta, R. W. Vadose Zone: Science and Technology Solutions; Battelle Press: Columbus OH, 2000. (3) Langmuir, D. Aqueous Geochemistry of Uranium. In Aqueous Environmental Chemistry; McConnin, R., Ed.; Prentice-Hall: Upper Saddle River, NJ, 1997; pp 494-512. (4) Runnells, D. D. Basic Contaminant Fate and Transport Processes in the Vadose Zone - Inorganics. In Handbook of Vadose Zone Characterization and Monitoring; Wilson, L. G., Everett, L. G., Cullen, S. J., Eds.; Geraghty & Miller Environmental Science and Engineering Series: 1995.

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