Lesson 4: Landfill Cover and Liner Systems for Water Quality Protection

This is the fourth lesson in the independent learning correspondence course on municipal solid waste (MSW) landfills. One lesson in this 12-part series will be published in Waste Age magazine each month throughout the year. Persons enrolled in the course need to take the first exam after completing this lesson.

If you are interested in taking the course for two continuing education credits (CEUs), send a check (payable to the University of Wisconsin) for $149 to Phil O''Leary, Department of Engineering Professional Development, University of Wisconsin, 432 N. Lake Street, Madison, WI 53706. Phone (608) 262-0493. E-mail:[email protected]. Website:www.wasteage.com.

Course registration can occur at anytime until December 2006. Previous lessons will be sent to you.

Click here to download PDF of Course 4 with figures. (Requires Adobe Acrobat Reader)

Successful landfill designs should control water movement to protect the environment, directing it away from the disposal area and containing it if it enters the landfill.

How Water Enters a Landfill

Drainage patterns around and within landfills must be carefully evaluated to effectively control water movement. Landfills located in areas where water might enter from surrounding surface drainage patterns must be carefully evaluated. When a landfill is constructed in a flat, open area, the entrance of surface drainage into the landfill is minimized. However, landfills often are located on sloping or valley topographies that cause water to flow downhill and, if not carefully managed, into the landfill.

Water that falls directly onto the landfill's surface while the landfill is open and after closure also must be managed. This water either will pass through the top of the landfill and into the waste, run off of the landfill, or evaporate. Controlling these natural processes is critical to achieving environmentally sound and operationally effective landfills. Consequently, the characteristics of the cover that is placed over the waste during operational phases and after closure will determine how much water enters the landfill.

Consequences of Uncontrolled Water Movement

Water that enters the landfill will move downward through the waste through the path of least resistance, becoming highly contaminated. A portion of the water may move downward until it encounters an impermeable layer, at which time it moves horizontally (laterally) and exits through the landfill's sideslope. The contaminated water will be discharged into the surface water system and could be extremely detrimental to nearby stream and lake water quality. Additionally, this can cause serious odor problems and likely will kill landfill cover vegetation.

Water that moves to the landfill's base, defined as leachate, either will be contained at the base or will leak into subsurface formations [See “Water Movement In and Around a Landfill” on page 124]. This water, if allowed to reach the groundwater flow system, will contaminate the water resource, rendering it unsuitable for drinking water.

Considerations in Facility Siting

When selecting the landfill location, managers must consider the surface topography and subsurface hydrogeologic conditions. Surface topography will influence how much effort will be necessary to control water from entering the site. For example, a landfill located adjacent to the base of a steep slope or in a ravine must have protective drainage control structures constructed above the landfill's perimeter. Landfills located on top of a rolling topography, on the other hand, must have drainage control structures constructed immediately below the fill to intercept any water run-off.

The best setting uses natural drainage patterns to carry stormwater runoff away from the landfill working face.

Managers also must consider subsurface geologic conditions and formations, such as the presence of clay, gravel, sand or fractured bedrock, as well as the depth to the groundwater table. Subsurface formations will influence the development of the liner system that is placed at the landfill's base and will determine the necessary subsurface monitoring system type. More information about landfill siting will be provided in Lesson 5.

The potential for groundwater contamination is directly related to the leachate amount generated, the type and effectiveness of the liner that is placed at the landfill base, and the subsurface formation characteristics. Landfills can be constructed in various settings, but the cover, liner and leachate management system must be coordinated with the hydrogeologic setting to provide adequate groundwater resource protection.

Leachate Management

To properly manage the leachate that reaches the landfill's base, it generally is necessary to install a leachate management system. This consists of a series of perforated pipes that will collect the leachate and direct it toward a storage reservoir or processing system. Almost all new landfills in the United States, except in particularly dry areas, have leachate collection systems [See “Leachate Collection System Layout” above].

When developing the collection system, consider the pipe size; drainage material, such as gravel or geosynthetics placed around the pipe; drainage media that is between the waste and the liner that allows the leachate to move laterally toward the pipes; and the pipe length and slopes. The goal is provide conditions that allow the leachate to quickly flow to the collection system.

Gravity will push leachate to a sump where it is removed by a pump. Generally, leachate collection pipes should not be placed through the side of the landfill because they can cause liner leaks.

Most often, collected leachate is directed to a municipal sewage plant for processing. Leachate is highly contaminated and, as such, must be treated before it is released into a surface water resource. When a landfill cannot connect through a sewer or truck its leachate to a municipal wastewater treatment plant, a treatment system must be constructed. These are highly specialized wastewater processing plants that must be carefully designed.

Several biological and physical chemical processes are available to process leachate. Leachate treatment systems likely will have high operating costs and can be difficult to maintain.

Cover Systems

A variety of soil and manufactured materials are available for final landfill covers. These materials must retard downward water movement, while simultaneously allowing for vegetation development and managing any lateral movement of infiltrating water. The cover design is regulated by federal and state standards [See a soil and geomembrane cover design and a complete geosynthetic cover in “Landfill Covers” on page 126]. The composite cover shown on page 126 is as specified by U.S. Environmental Protection Agency (EPA) standards. The pictured geosynthetic cover must be constructed to provide the same level of protection as provided by the federally specified standard.

It is common for individual states to have their own cover standards, but they must, at a minimum, meet the federal standards. Certain standard exemptions are available, depending on local hydrologic conditions. Design variations are most notably implemented in arid regions.

The basic operating principle for a final landfill cover is to minimize downward infiltration while allowing water to evaporate into the atmosphere through evapotranspiration, which occurs in plants. The cover also must allow water to move laterally through the topsoil formation. This lateral movement minimizes the potential for slope failure.

If a large amount of water accumulates within the cover above the impermeable layer, there is a greater likelihood of a landslide of the topsoil material.

Typically, the impermeable soil layer is specified to minimize water infiltration. This specification is described as the hydraulic conductivity and generally is 10-7 centimeters per second (cm/sec). Laboratory tests are available to determine the cover material's hydraulic conductivity. Most often, a clay or silty clay soil is necessary to meet this standard.

However, a new technology developed at the University of Wisconsin allows covers to be constructed entirely from soil. In arid regions, it is possible to use the soil's physical properties to minimize infiltration through the cover equal to geomembrane cover construction. The principal behind this is to use layers of soil materials that minimize downward water movement while maximizing evapotranspiration from the cover. These alternate earthen covers have been shown to be as effective as the covers constructed with geomembranes and clays at a much lower cost.

Several geosynthetic membranes and drainage media also are available for cover construction. The geosynthetic cover pictured above is typical of this construction type, but it is not the only available alternative. A number of manufacturers supply these materials.

Liner Systems

During a landfill's initial siting and development, the type of liner system that will be placed at the base must be determined. Federal regulations specify that the liner system be constructed according to the composite liner diagram shown below [on left], or that the construction be equivalent. The double membrane liner is an example of an equivalent liner.

Each layer in the liner system has its own function. The bottom clay layer minimizes downward water movement, to the maximum extent possible, into the soil material. This material also is expected to provide a firm base so that the remaining liners can be installed. The geomembrane liner limits water, to the maximum extent possible, into the clay liner.

Together, the geomembrane and clay are very effective in minimizing leachate leakage. For example, if a hole develops in the geomembrane, the amount of leachate that will pass through the hole is minimized by the impermeability of the clay.

The drainage layer above the geomembrane allows the leachate to flow toward the leachate collection system. This drainage layer must be constructed of materials that allow the water to move laterally and must not exceed a depth of one foot above the liner. Above the drainage layer, it is often standard operating procedure to place an initial lift of waste to protect the drainage and liner layers as landfill operations begin.

Leachate collection lines are placed within trenches that are constructed as part of the liner system. The leachate collection pipes slope toward the collection sump, at which point the leachate is removed from the landfill. The pipes must be structurally sound, yet have holes that allow the leachate to flow into the lines. Leachate lines should be periodically cleaned with sewer cleaning type machines.

Seismic and Slope Stability

When developing landfill liners and covers, it is important to allow for instabilities that may develop during earthquakes or that are associated with slope failure. Calculations can predict the forces on the liner and cover that may result during an earthquake. Even in areas where earthquakes are not concerns, the forces that the liner system will encounter should be studied.

Waste pushing down on the liner where the liner is constructed on a slope, such as on the side of a landfill, can cause the liner system to fail. Consequently, consider the potential for slope failure when selecting materials, determining the angle of the landfill's interior sideslopes and measuring the friction between the various materials that make up the liner system. Taking slope failure into account initially can ensure it won't occur once waste is landfilled. Several very large failures have taken place in recent years, and preventing slope failure has been identified as an important factor in landfill development and operation.

In addition to considering slope failures that may occur on the liner, three other failure types must be taken into account (additional references will be available in the study guide).

First, the cover system may slide off a portion of the landfill. This results when managers do not adequately take into account the grades, forces and friction angles within the cover system.

The second failure type occurs when waste, like an avalanche, falls and slides sideways out of the landfill.

Rotational failure is the third concern and occurs where waste is forced out the side of the landfill.

Predicting Performance

The most common mathematical formula used to predict landfill leachate generation is the HELP (Hydrologic Evaluation of Landfill Performance) Model [See “Landfill Water Balance” for a depiction of the factors that go into the HELP Model and an example of the results on page 128]. This computer program, available from the EPA, takes into account the amount of precipitation, temperature, weather conditions that control evapotranspiration, cover design characteristics, waste characteristics and liner permeability to predict the leachate quantity that will be generated, and to estimate the potential amount of leakage from the base of the landfill.

Other computer models are available to assess how groundwater contaminants may spread within the subsurface flow system. These models must account for the chemical and biological reactions from pollutants moving through the groundwater.

Using groundwater models can be challenging because a large amount of data is necessary, and many assumptions must go into setting up the model. Groundwater model results typically are concentration maps that show the contaminant downgradient from the landfill.

The best use of both the groundwater and HELP computer model is to assess alternative practices that could be implemented at the landfill.

Monitoring Performance

A sanitary landfill is a highly complex biological and chemical processing and storage facility. As such, information needs to be continuously collected to monitor the landfill's ability to manage waste and its potential environmental impact. The most common landfill monitoring device is the groundwater monitoring well. This, together with the gas monitoring probe that was described in Lesson 3, provides landfill managers with a picture of the contaminants that may be escaping from the site.

In addition to monitoring groundwater and gas, it also is appropriate to periodically assess landfill conditions including erosion, vegetative stress, the potential escape of leachate through the landfill cover, and the escape of odors. Federal regulations require continuous groundwater monitoring for the life of the landfill, and for 30 years after its closure.

Long-Term Maintenance

The facilities within the landfill, while constructed to withstand the forces of nature for a long period, must be periodically evaluated and appropriately maintained. For example, high volumes of rainwater runoff may displace drainage structures, gas migrating through the cover may kill some of the grass and erosion may occur.

Repairing these kinds of conditions while operating the leachate management system and the gas management system are necessary long-term landfill requirements. Currently, federal regulations, and most state regulations, dictate that long-term care maintenance must be provided by the owner of the landfill for 30 years after closure.

Financial resources also must be available to guarantee that money will be available for closure and long-term care. This money can be equivalent to up to 20 or 30 percent of the overall cost of developing, operating and closing the landfill.

In May, Lesson 5, “Evaluating Potential Sanitary Landfill Sites,” will describe landfill siting issues.

Phil O'Leary and Patrick Walsh are solid waste specialists at the University of Wisconsin-Madison.

Projected Average Annual Totals for 20 Years Inches
Precipitation 35.37
Runoff from cover 0.321
Evapotranspiration from cover 26.342
Lateral drainage from cap drainage layer 0.0005
Percolation through landfill clay cap layer 8.6668
Leachate collected from drainage layer above landfill liner 7.1290
Leachate percolation from the composite liner 0.1458
Source: D.G. Fenn et al., The Use of the Water Balance Method for Predicting Leachate Generation from Solid Waste Disposal Sites, 1975.