Landfill Caps and Enhancements


Landfill capping is a containment technology that forms a barrier between the contaminated media and the surface, thereby shielding humans and the environment from the harmful effects of its contents and perhaps limiting the migration of the contents. A cap must restrict surface water infiltration into the contaminated subsurface to reduce the potential for contaminants to leach from the site.

Landfills have been the most common form of waste disposal, and old landfills are present or in close proximity to most communities in the United States. Prior to environmental laws that regulated waste disposal, landfills were no more than holes in the ground filled with waste and covered with dirt. Many of the old landfills that are problems today were constructed in this fashion. When hazardous waste was disposed, it often was placed in metal drums that rusted through in a few years, leaving the waste to seep through the landfill. Water was allowed to seep through the cover of the landfill, saturate the wastes, and come out the bottom or sides as leachate (i.e., liquid that leaches through the landfill and collects in pockets in the landfill material or below the landfill). Leachate, in turn, often flowed into groundwater.

In 1976, Congress passed the Resource Conservation and Recovery Act (RCRA), tightening the regulatory oversight of existing landfills and establishing basic standards for covering landfills (i.e., landfill caps and containing leachate. Two types of caps were required: those for hazardous waste landfills and those for non-hazardous waste landfills. The former type of cap consists of three layers: 1) an upper vegetative (topsoil) layer; 2) a drainage layer; and 3) a low permeability layer made of a synthetic material covering two feet of compacted clay. The latter is less stringent, designed in conjunction with the bottom liner system to keep wastes from migrating. Alternative designs may be considered, but they must be equivalent in performance to the standard caps.

Cap performance varies, depending upon its function and where it is used. For example, compacted clay liners are effective if they retain a certain moisture content, but they are susceptible to cracking if the clay material is dried out. Therefore, EPA has been investigating four types of "alternative" landfill covers under its Alternative Cover Assessment Program (ACAP): evapotranspiration (ET) covers, capillary barrier ET covers, asphalt covers, and bioengineering management covers. ET covers, the most common of the alternatives, minimize downward migration of water by using soil and vegetation to store and transpire water before it enters the contaminated fill.

Limitations and Concerns

Landfill caps do not lessen the toxicity, mobility, or volume of hazardous wastes, but they do limit migration. Landfill caps are most effective where most of the underlying waste is above the water table.

Landfill cap integrity must not be compromised by present and future land use activities. Institutional controls are often required to protect the landfill.

Landfill caps have a limited life span. They are estimated to last from 50 to 100 years. In areas with high rates of subsidence and regions prone to earthquakes, the cap and its foundation should be designed appropriately. Changes in conditions, such as soil moisture and earth movement, should be monitored as indicators of potential problems and mitigated before hazardous releases occur. Fluctuations in air temperature and precipitation may also affect the cap's integrity by causing cracking or erosion. In addition, plant roots and burrowing animals can undermine the cap's integrity.

ET covers must be planned using natural vegetation, and soil depth should be adequate to ensure that water does not come into contact with the landfill materials. A challenge is getting plants started (as irrigation is a potential problem), and incorporating a long-term monitoring system to ensure that water does not come into contact with contaminants and that plant life survives.

These considerations are particularly important for containing radioactive waste because of the long-term isolation required. In order to promote the cap's longevity, infiltration barriers should be covered by a soil layer sufficiently thick to extend below the frost line, to accommodate the rooting depths of native plants, and to extend below the probable depth of animal burrows. The long-term durability of cap materials must be assured when the purpose of the cap is to isolate radioactive waste. For example, high density polyethylene (HDPE) geo-membranes are susceptible to degradation from sunlight as well as chemical and biological degradation. However, these degradation mechanisms are generally eliminated by burial of the membrane in cover systems that are three meters in depth, thus increasing the longevity of the geo-membrane.

While construction of the geo-membrane liner typically provides an impermeable barrier, damage to the liner, in the form of small tears or punctures, may occur when the protective soil cover is placed over the liner, thereby affecting its integrity and reliability.

Caps for radium-contaminated sites must be designed to confine gaseous radon until it has essentially decayed. Ensuring that the depth of cover is a certain amount will accomplish this. In addition, a soil cover is required for gamma radiation shielding.

A cap, by itself, does not prevent the horizontal flow of groundwater through waste; it only controls the vertical entry of water into the waste.

A cap must extend a few feet beyond the perimeter of the contaminated area to prevent the lateral infiltration of rain.

Compacted clay liners are effective if they retain a certain moisture content, but they are susceptible to cracking if the clay material is desiccated. As a result, alternate cap designs are usually considered for arid environments.

The dry barrier enhancement has difficulty when the cover is subjected to sustained precipitation. To intercept all of the water, the air flow rate must be relatively great.


Landfill caps and their enhancements are used to contain a variety of materials. They may be temporary or final. Temporary caps can be installed before final closure to minimize the generation of leachate until a better remedy is implemented.

Technology Development Status

Landfill caps are commercially available. Alternatives, such as caps designed for dry climates and enhancements, are being field tested.

Web Links (landfill cap enhancements) (alternative designs) (alternative designs)

Other Resources and Demonstrations

See for a web site that has description of all projects under its ACAP program. These include 47 demonstrations and 46 full-scale projects.

In 1996, the U.S. Department of Energy/Grand Junction Project Office (DOE/GJPO), the State of Utah, and the U.S. Environmental Protection Agency (EPA) collaborated in the construction of a cover to contain 2.5 million cubic yards of radioactive material removed from the Monticello Mill Superfund site. The large, multi-layered cover combined fundamental ecological and engineering principles. Groundwater recharge is limited naturally at Monticello, where thick soils store precipitation until evaporation and plant transpiration (evapotranspiration) seasonally return it to the atmosphere. The cover design mimics and enhances this natural water balance. A capillary barrier underlying a thick soil "sponge" enhances water storage and prevents downward unsaturated flow. The cover also was designed to control radon flux, bio-intrusion, and erosion, and to protect critical layer interfaces from frost.

See and for guidance on bioreactor landfills.