Description:

• 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.
• 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 material 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 its passage 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 geosynthetic clay liners (GCLs). 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 mil), widths (15 to 100 ft), and lengths (180 to 840 ft). The candidate list of polymers commonly used is lengthy, which includes polyvinyl chloride (PVC), polyethylenes of various densities, reinforced chlorosulfonated polyethylene (CSPE-R), polypropylene, ethylene interpolymer alloy (EIA), and many newcomers. Soils used as barrier materials generally are clays that are compacted to a hydraulic conductivity no greater than 1 x 10^-6 cm/sec. Compacted soil barriers are generally installed in 6-inch minimum 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.

For facilities on top of putrescible wastes, the collection and control of methane and carbon dioxide, potent greenhouse gases, must be part of facility design and operation.

Asphalt/Concrete Cap

The most effective single-layer caps are composed of concrete or bituminous asphalt. It is used to form a surface barrier between landfill and the environment. An asphalt concrete cap would reduce leaching through the landfill into an adjacent aquifer.

RCRA Subtitle C Cap

The RCRA C multilayered landfill cap is a baseline design that is 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 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 Cap

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 1 x 10-5 cm/s, or equivalent permeability of any bottom liner or natural subsoils present, whichever is less.
• The infiltration layer must contain at least 45 cm of earthen material.
• The erosion control layer must be at least 15 cm 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.

Synonyms:

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 ground water 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:

Laboratory tests are also needed to ensure that geosynthetic materials will meet the cap requirements, For example, geosynthetics in caps may be subjected to tensile stresses caused by subsidence and by the gravitational tendency of a geomembrane or material adjacent to it to slide or be pulled down slopes.

Since facility performance is a function of quality construction more so than selection of materials, construction quality assurance of caps are critical. EPA has generated a technical guidance document on this subject. The technical guidance should be strictly followed during design and construction.

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 ground water. Performance can be monitored much more effectively by including pan lysimeter in future caps.

Cost:

Landfill caps are generally the least expensive way to manage the human health and ecological risks effectively. Rough industry cost are $175k/acre for RCRA Subtitle D, and $225k/acre for RCRA Subtitle C.

Additional cost information can be found in the Hazardous, Toxic, and Radioactive Wastes (HTRW) Historical Cost Analysis System (HCAS) developed by Environmental Historical Cost Committee of Interagency Cost Estimation Group.

References:

Innovative Remediation Technologies:  Field Scale Demonstration Project in North America, 2nd Edition

Abstracts of Remediation Case Studies, Volume 4,  June, 2000, EPA 542-R-00-006

Guide to Documenting and Managing Cost and Performance Information for Remediation Projects - Revised Version, October, 1998, EPA 542-B-98-007

DOE, 1995. Technology Catalogue, Second Edition, Office of Environmental Management and Office of Technology Development, DOE/EM-0235.

EPA, 1987. Geosynthetic Design Guidance for Hazardous Waste Landfill Cells and Surface Impoundments, EPA/600/2-87/025.

EPA, 1988. Technology Screening Guide for Treatment of CERCLA Soil and Sludge, EPA/540/288/004.

EPA, 1989. Final Covers on Hazardous Waste Landfills and Surface Impoundments, Technical Guidance Document, Office of Solid Waste and Emergency Response, Washington, DC, EPA/530/SW-89/047.

EPA, 1991. Compilation of Information on Alternative Barriers for Liner and Cover Systems, EPA/600/2-91/002.

EPA, 1991. Inspection Techniques for the Fabrication of Geomembrane Field Seams, Technical Guidance Document, EPA/530/SW-91/051.

EPA, 1993. Construction Quality Control and Quality Assurance at Waste Containment Facilities, Technical Guidance Document, Office of Research and Development, RREL, Cincinnati, OH, EPA/600/R-93/182.

EPA, 1993. Engineering Bulletin Landfill Covers, EPA/540/S-93/500.

EPA, 1995. RCRA Subtitle D (258) Seismic Design Guidance for Municipal Solid Waste Landfills, Office of Research and Development, NRMRL, Cincinnati, OH, EPA/600/R-95/051.

EPA, 1995. Report of 1995 Workshop on Geosynthetic Clay Liners, Office of Research and Development, NRMRL, Cincinnati, OH, EPA/600/R-96/149.

EPA, 1997. Best Management Practices (BMPs) for Soil Treatment Technologies: Suggested Operational Guidelines to Prevent Cross-media Transfer of Contaminants During Clean-UP Activities, EPA OSWER, EPA/530/R-97/007.

EPA, 1997. Technology Alternatives for the Remediation of Soils Contaminated with As, Cd, Cr, Hg, and Pb, Engineering Bulletin, EPA540/R-97/008.

EPA, 1998. Evaluation of Subsurface Engineered Barriers at Waste Sites, Technology Report, EPA/542/R-98/005.

AFCEE/ERT, 1999. Landfill Covers for Use at Air Force Installations.

Federal Remediation Technologies Roundtable, 1998. Remediation Case Studies: In Situ Soil Treatment Technologies (Soil Vapor Extraction, Thermal Processes), EPA/542/R-98/012

• Soil Vapor Extraction at the Seymour Recycling Corporation Superfund Site, Seymour, Indiana

Federal Remediation Technologies Roundtable, 1998. Remediation Case Studies: Groundwater Pump and Treat (Nonchlorinated Solvents), EPA/542/R-98/014

• Pump and Treat and Containment of Contaminated Groundwater at the Sylvester/Gilson Road Superfund Site, Nashua, New Hampshire

Federal Remediation Technologies Roundtable, 1998. Remediation Case Studies: Debris and Surface Cleaning Technologies, and Other Miscellaneous Technologies, EPA/542/R-98/017.

• Lawrence Livermore National Laboratory (LLNL) Site 300 - Pit 6 Landfill Operable Unit (OU), Livermore, CA.

• AFCEE action, Fairchild AFB, WA
• DOE Oak Ridge, TN facility
• Lawrence Livermore National Laboratory, Site 300, Coast Ranges, CA
• DOE Demo, Lee Acres landfill, Farmington, NM
• Lawrence Livermore National Laboratory, Site 300, Pit 6 Landfill OU, Livermore, CA
• Seymour Recycling Corporation Superfund Site, Seymour, IN
• Sylvester/Gilson Road Superfund Site, Nashua, NH

General FRTR Agency Contacts

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A list of vendors offering Soil Containment Treatment is available is from EPA REACH IT which combines information from three established EPA databases, the Vendor Information System for Innovative Treatment Technologies (VISITT), the Vendor Field Analytical and Characterization Technologies System (Vendor FACTS), and the Innovative Treatment Technologies (ITT), to give users access to comprehensive information about treatment and characterization technologies and their applications.

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