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Soil Pile Disposition Assessment
Section 6 - Potential Selection and Limitations of the Corrective Measure(s) Technologies for Soil
Preliminary selection of remedial technologies which were determined to have the best potential on the basis of effectiveness, implementability, and cost include capping; excavation, with off-site disposal; solidification/stabilization; soil washing/density separation; and phytoremediation. A general discussion of the preliminary selected remedial technologies is presented below.
Source containment technologies involve contaminated sites which are capped with low permeability clays, geomembrane liners, asphalt, concrete or a combination to prevent surface water infiltration and creation of leachate. The source containment technologies are referred to as �landfill caps�. Landfill caps are usually designed to meet RCRA standards for either Municipal landfills (RCRA Subtitle D) or Industrial landfills (RCRA Subtitle C).
Typical landfill caps can be used to:
Minimize exposure on the surface of the waste facility;
Prevent vertical infiltration of water into wastes that would create contaminated leachate;
Contain waste while treatment is being applied;
Control gas emissions from underlying waste; and
Create a land surface that can support vegetation and/or be used for other purposes.
Landfill capping is the most common form of remediation because it is generally less expensive than other technologies and effectively manages the human and ecological risks associated with a remediation site.
The design of landfill caps is site specific and depends on the intended functions of the system. Landfill caps can range from a one-layer system of vegetated soil to a complex multi-layer system of soils and geosynthetics. In general, less complex systems are required in dry climates and more complex systems are required in wet climates. The materials used in the construction of landfill caps include low-permeability and high-permeability soils and low-permeability geosynthetic products. The low-permeability materials divert water and prevent vertical infiltration into the waste. The high permeability materials carry water away that percolates into the cap. Other materials may be used to increase slope stability.
The most critical components of a landfill cap are the barrier layer and the drainage layer. The barrier layer can be low-permeability soil (clay) and/or a geosynthetic clay liner. A flexible geomembrane liner is placed on top of the barrier layer. Geomembranes are usually supplied in large rolls and are available in several thickness (20 to 140 millimeters), widths (15 to 100 feet), and lengths (180 to 840 feet). The candidate list of polymers commonly used is lengthy and includes polyvinyl chloride (PVC), polyethylenes of various densities, reinforced polyethylene, polypropylene, ethylene interpolymer alloy, and many newcomers. Soils used as barrier materials generally are clays compacted to a hydraulic conductivity no greater than 10‑6 centimeters/second. At a minimum, compacted soil barriers are generally installed in 6-inch lifts to achieve a thickness of 2 feet or more. A composite barrier uses both soil and a geomembrane, taking advantage of the properties of each. The geomembrane is essentially impermeable, but, if it develops a leak, the soil component prevents significant leakage into the underlying waste.
The most effective single-layer caps are composed of concrete or bituminous asphalt. It is used to form a surface barrier between the landfill and the environment. An asphalt or concrete cap would reduce leaching through the landfill into an adjacent aquifer.
The RCRA Subtitle C multilayered landfill cap is a baseline design suggested for use in RCRA hazardous waste applications. These caps generally consist of an upper vegetative (topsoil) layer, a drainage layer, and a low permeability layer which consists of a synthetic liner of over 2 feet of compacted clay. The compacted clay liners are effective if they retain a certain moisture content but are susceptible to cracking if the clay material is desiccated. As a result, alternate cap designs are usually considered for arid environments.
RCRA Subtitle D requirements are for non-hazardous waste landfills. The design of a landfill cover for a RCRA Subtitle D facility is generally a function of the bottom liner system or natural subsoils present. The cover must meet the following specifications:
The material must have a permeability no greater than 10-5 centimeters/second, or equivalent permeability of any bottom liner or natural subsoils present, whichever is less.
The infiltration layer must contain at least 45 centimeters of earthen material.
The erosion control layer must be at least 15 centimeters of earthen material capable of sustaining native plant growth.
Alternative design can be considered, but must be of equivalent performance as the specifications outlined above. All covers should be designed to prevent the �bathtub� effect. The bathtub effect occurs when a more permeable cover is placed over a less permeable bottom liner or natural subsoil. The landfill then fills up like a bathtub.
| Capping | |
| Applicability: | Landfill caps may be temporary or final. Temporary caps can be installed before final closure to minimize generation of leachate until a better remedy is selected. They are usually used to minimize infiltration when the underlying waste mass is undergoing settling. A more stable base will thus be provided for the final cover, reducing the cost of the post-closure maintenance. Landfill caps also may be applied to waste masses that are so large that other treatment is impractical. At mining sites for example, caps can be used to minimize the infiltration of water to contaminated tailings piles and to provide a suitable base for the establishment of vegetation. In conjunction with water diversion and detention structures, landfill caps may be designed to route surface water away from the waste area while minimizing erosion. |
| Limitations: | Landfilling does not lessen toxicity, mobility, or volume of hazardous wastes, but does mitigate migration. Landfill caps are most effective where most of the underlying waste is above the water table. A cap, by itself, cannot prevent the horizontal flow of groundwater through the waste, only the vertical entry of water into the waste. In many cases landfill caps are used in conjunction with vertical walls to minimize horizontal flow and migration. The effective life of landfill components (including cap) can be extended by long-term inspection and maintenance. Vegetation, which has a tendency for deep root penetration, must be eliminated from the cap area. In addition, precautions must be taken to assume that the integrity of the cap is not compromised by land use activities. |
| Data Needs: | Many laboratory tests are needed to ensure that the materials being considered for each of the landfill cap components are suitable. Tests to determine the suitability of soil include grain size analysis, Atterberg limits, and compaction characteristics. Landfill instability can be solved by understanding interface friction properties between all material layers, natural or synthetic. The major engineering soil properties that must be defined are the shear strength and hydraulic conductivity. Shear strength may be determined with the unconfined compression test, direct shear test, or triaxial compression test. Hydraulic conductivity of soils may be measured in the laboratory by the constant head permeability test or the falling head permeability test. Field hydraulic conductivity tests on test pads are generally recommended prior to actual cover construction to ensure that the low-permeability requirements can actually be met under construction conditions. |
| Performance Data: | Previously installed caps are hard to monitor for performance. Monitoring well systems or infiltration monitoring systems can provide some information, but it is often not possible to determine whether the water or leachate originated as surface water or groundwater. Performance can be monitored much more effectively by including pan lysimeters in future caps. |
| Cost: | Landfill caps are generally the least expensive way to manage the human health and ecological risks effectively. Rough industry costs are $175/acre for RCRA Subtitle D, and $225/acre for RCRA Subtitle C. |
Contaminated material is removed and transported to permitted off-site treatment and/or disposal facilities. Some pretreatment of the contaminated media usually is required in order to meet land disposal restrictions.
| Source Removal | ||
| Applicability: | Excavation and off-site disposal is applicable to the complete range of contaminant groups with no particular target group. Excavation and off-site by relocating the waste to a different (and presumably safer) site. | |
| Limitations: | Factors that may limit the applicability and effectiveness of the process include: | Generation of fugitive emissions may be a problem during operations. | The distance from the contaminated site to the nearest disposal facility with the required permit(s) will affect cost. | Depth and composition of the media requiring excavation must be considered. | Transportation of the soil through populated areas may affect community acceptability. |
Data Needs:
The type of contaminant and its concentration will impact off-site disposal requirements. Soil characterization as dictated by land disposal restrictions (LDRs) is required. Most hazardous wastes must be treated to meet either RCRA or non-RCRA treatment standards prior to land disposal.
Cost:
Cost estimates for excavation and disposal range from $15 to $65 per cubic yard depending on the nature of waste (i.e., non-hazardous waste classification) and methods of excavation. These estimates include excavation/removal, transportation, and disposal at a RCRA permitted facility. Additional cost of treatment at disposal facility may also be required. Excavation and off-site disposal is a relatively simple process, with proven procedures. It is a labor-intensive practice with little potential for further automation. Additional costs may include soil characterization and treatment to meet land ban requirements and costs to backfill any excavated areas back to the original grade.
6.3 - Ex situ Treatment Technologies
6.3.1 Stabilization/Solidification
Ex situ stabilization/solidification (S/S) contaminants are physically bound or enclosed within a stabilized mass (solidification), or chemical reactions are induced between the stabilizing agent and contaminants to reduce their mobility (stabilization). Ex situ S/S, however, typically requires disposal of the resultant materials. Under CERCLA, material can be replaced on-site.
There are many innovations in the stabilization and solidification technology. Most of the innovations are modifications of proven processes directed to encapsulation or immobilization of the harmful constituents, which involve processing of the waste or contaminated soil. Nine distinct innovative processes or groups of processes include: (1) bituminization, (2) emulsified asphalt, (3) modified sulfur cement, (4) polyethylene extrusion, (5) pozzolan/Portland cement, (6) radioactive waste solidification, (7) sludge stabilization, (8) soluble phosphates, and (9) vitrification/molten glass.
Typical ex situ S/S is a short- to medium-term technology.
| Stabilization/Solidification | ||
| Applicability: | The target contaminant group for ex situ S/S is inorganics, including radionuclides. Most S/S technologies have limited effectiveness with organics and pesticides. | |
| Limitations: | Environmental conditions may affect the long-term immobilization of contaminants. | Some processes result in a significant increase in volume (up to double the original volume). | Certain wastes are incompatible with different processes. Treatability studies are generally required. | Organics are generally not immobilized. | Long-term effectiveness has not been demonstrated for many contaminant/process combinations. |
Data Needs:
Soil parameters that may be required include particle size, Atterberg limits, moisture content, metal concentrations, sulfate content, organic content, density, permeability, unconfined compressive strength, leachability, microstructure analysis, and physical and chemical durability.
Cost:
Ex situ solidification/stabilization processes are among the most mature remediation technologies. Representative overall costs from a previous field study effort at CSSA indicate an approximate cost of under $100 per ton, including excavation.
Bituminization
In the bituminization process, wastes are embedded in molten bitumen and encapsulated when the bitumen cools. The process combines heated bitumen and a concentrate of the waste material, usually in slurry form, in a heated extruder containing screws that mix the bitumen and waste. Water is evaporated from the mixture to about 0.5% moisture. The final product is a homogenous mixture of extruded solids and bitumen.
Emulsified Asphalt
Asphalt emulsions are very fine droplets of asphalt dispersed in water that are stabilized by chemical emulsifying agents. The emulsions are available as either cationic or anionic emulsions. The emulsified asphalt process involves adding emulsified asphalts having the appropriate charge to hydrophilic liquid or semi-liquid wastes at ambient temperature. After mixing, the emulsion breaks, the water in the waste is released, and the organic phase forms a continuous matrix of hydrophobic asphalt around the waste solids. In some cases, additional neutralizing agents, such as lime or gypsum, may be required. After given sufficient time to set and cure, the waste is uniformly distributed throughout the resulting solid asphalt and is impermeable to water.
Modified Sulfur Cement
Modified sulfur cement is a commercially-available thermoplastic material. It is easily melted (127� to 149� C (260� to 300� F) and then mixed with the waste to form a homogenous molten slurry which is discharged to suitable containers for cooling, storage, and disposal. A variety of common mixing devices such as paddle mixers and pug mills, can be used. The relatively low temperatures limit emissions of sulfur dioxide and hydrogen sulfide to allowable threshold values.
Polyethylene Extrusion
The polyethylene extrusion process involves the mixing of polyethylene binders and dry waste materials using a heated cylinder containing a mixing/transport screw. The heated, homogenous mixture exits the cylinder through an output die into a mold, where it cools and solidifies. Polyethylene�s properties produce a very stable, solidified product. The process has been tested on nitrate salt wastes at plant-scale, establishing its viability, and on various other wastes at the bench and pilot scale.
Pozzolan/Portland Cement
Pozzolan/Portland cement process consists primarily of silicates from pozzolanic-based materials like fly ash, kiln dust, pumice, or blast furnace slag and cement-based materials like Portland cement. These materials chemically react with water to form a solid cementious matrix which improves the handling and physical characteristics of the waste. They also raise the pH of the water which may help precipitate and immobilize some heavy metal contaminants. Pozzolanic and cement-based binding agents are typically appropriate for inorganic contaminants. The effectiveness of this binding agent with organic contaminants varies.
Soluble Phosphates
The soluble phosphates process involves the addition of various forms of phosphate and alkali for control of pH as well as for formation of complex metal molecules of low-solubility to immobilize (insolubilize) the metals over a wide pH range. Unlike most other stabilization processes, soluble phosphate processes do not convert the waste into a hardened, monolithic mass. One application of soluble phosphates include the use of Phosphate Induced Metal Stabilization (PIMS) which utilizes a patented Apatite II mineral generated from fish bones to provide the soluble phosphate and the nucleating site for the conversion of lead to lead phosphate. This technology is currently being field tested at CSSA on the SWMU B-20/21 sifted soils. Results of the field study are expected to be reported in a final report due September 2003.
6.3.2 Soil Washing/Density Separation
Soil washing is a water-based process for scrubbing soils ex situ to remove contaminants. The process removes contaminants from soils in one of two ways:
By dissolving or suspending soil particles in the wash solution (which can be sustained by chemical manipulation of pH for a period of time).
By concentrating them into a smaller volume of soil through particle size separation, gravity separation, and attrition scrubbing (similar to those techniques used in sand and gravel operations).
Soil washing systems that incorporate most of the removal techniques offer the greatest promise for application to soils contaminated with a wide variety of heavy metal, radionuclides, and organic contaminants. Commercialization of the process, however, is not yet extensive.
The concept of reducing soil contamination through the use of particle size separation is based on the finding that most organic and inorganic contaminants tend to bind, either chemically or physically, to clay, silt, and organic soil particles. The silt and clay, in turn, are attached to sand and gravel particles by physical processes, primarily adhesion. Washing processes that separate the fine (small) clay and silt particles from the coarser sand and gravel soil particles effectively separate and concentrate the contaminants into a smaller volume of soil that can be further treated or disposed. Gravity separation is effective for removing high or low specific gravity particles such as heavy metal-containing compounds (lead, radium oxide, etc.). Attrition scrubbing removes adherent contaminant films from coarser particles. However, attrition washing can increase the fines in soils processed. The clean, larger fraction can be returned to the site for continued use.
Complex mixture of contaminants in the soil (such as a mixture of metals, nonvolatile organics, and SVOCs) and heterogeneous contaminant compositions throughout the soil mixture make it difficult to formulate a single suitable washing solution that will consistently and reliably remove all of the different types of contaminants. For these cases, sequential washing, using different wash formulations and/or different soil-to-wash fluid ratios, may be required.
Soil washing is generally considered a media transfer technology. The contaminated water generated from soil washing is treated with the technology(s) suitable for the contaminants.
The duration of soil washing is typically short- to medium-term.
Density separation or particle size separation is a technology potentially applicable to soils containing metals. It is typically not applicable to soils with volatiles, since the processing would release the volatiles to the atmosphere, and VOCs do not typically adsorb strongly to soils.
Density separation involves excavation and physical size separation of soil particles through either a wet gravity separation process or the use of different size mesh sieves for a dry separation.
This process can be used to separate the fine particles from the larger particles. The contaminated fraction is then treated by the appropriate method. However, the volume that now requires additional treatment has been reduced.
| Density Separation | ||
| Applicability: | The contaminants potentially applicable to density separation are pesticides/PCBs, SVOCs, and/or metals. | |
| Limitations: | Development of secondary residuals, which require further treatment; | Often used as a part of an overall treatment process; | Limited in certain soil and site conditions; | Requires large staging areas; and | May result in liquids that require treatment. |
Data Needs:
Density separation for soils at CSSA has been shown in laboratory benchscale tests as being effective.
Cost:
The estimated costs for use of this technology provided through laboratory benchscale studies, is $60 per cubic yard.
Soil Washing
Applicability:
The target contaminant groups for soil washing are SVOCs, fuels, and heavy metals. The technology can be used on selected VOCs and pesticides. The technology offers the ability for recovery of metals and can clean a wide range of organic and inorganic contaminants from coarse-grained soils.
Limitations:
Complex waste mixtures (e.g., metals with organics) make formulating the washing fluid difficult.
High humic content in soil may require pretreatment.
The aqueous stream will require treatment at demobilization.
Additional treatment steps may be required to address hazardous levels of washing solvent remaining in the treated residuals.
It may be difficult to remove organics adsorbed onto clay-size particles.
Data Needs:
Particle size distribution (0.24 to 2 millimeter optimum range); soil type, physical form, handling properties, and moisture content; contaminant type and concentration; texture; organic content; cation exchange capacity; pH and buffering capacity. A complete bench scale treatability study should always be completed before applying this technology as a remedial solution.
Cost:
The average cost for use of this technology, including excavation, is approximately $170 per ton, depending on site-specific conditions and the target waste quantity and concentration.
6.4 - IN situ Treatment Technologies
Typical in situ phytoremediation is a set of processes that uses metal tolerant, hyperaccumulating plants to extract contamination from the soil media. There are several ways plants can be used for the phytoremediation. These mechanisms include phytoextraction and phytostabilization.
| Phytoremediation | ||
| Applicability: | Phytoremediation can be used to clean up metal contaminants from surface soils. Plants also produce enzymes, such as dehalogenase and oxygenase, which help catalyze degradation. | |
| Limitations: | It is limited to shallow soils. | High concentrations of hazardous materials can be toxic to plants. | Climatic or seasonal conditions may interfere or inhibit plant growth, slow remediation efforts, or increase the length of the treatment period. | It can transfer contamination across media, e.g., from soil to biomass. | The toxicity and bioavailability of biodegradation products is not always known. Products may be mobilized into groundwater or bioaccumulated in animals. More research is needed to determine the fate of various compounds in the plant metabolic cycle to ensure that plant droppings and products manufactured by plants do not contribute toxic or harmful chemicals into the food chain or increase risk exposure to the general public. |
Data Needs:
In addition, detailed information is needed to determine the kinds of soil used for phytoremediation projects. Water movement, reductive oxygen concentrations, root growth, and root structure all affect the growth of plants and should be considered when implementing phytoremediation.
Performance Data:
The U.S. Air Force used poplar trees to contain a groundwater TCE plume. TCE was degraded in the tissues of the poplar trees. The trees pumped a sufficient amount of water to produce a cone of depression limiting the spread of the TCE plume.
Cost:
Construction estimates for phytoremediation are $200K/acre and $20K/acre for operations and maintenance.
Enhanced rhizosphere biodegradation takes place in the soil surrounding plant roots. Natural substances released by plant roots supply nutrients to microorganisms, which enhances their ability to biodegrade organic contaminants. Plant roots also loosen the soil and then die, leaving paths for transport of water and aeration. This process tends to pull water to the surface zone and dry the lower saturated zones.
Phytostabilization, also referred to as in-place stabilization, is primarily used for the remediation of soil, sediment, and sludges using plant roots to limit contaminant mobility and bioavailability in the soil. The plants primary purposes are to:
Decrease the amount of water percolating through the soil matrix, which may result in the formation of a hazardous leachate;
Act as a barrier to prevent direct contact with contaminated soil; and
Prevent soil erosion and distribution of toxic metals to other areas.