LANDFILLS: New Landfill Cover Testing May Result In Reduced Costs

Bill Siuru

Looking for a suitable, cost-effective hazardous waste landfill cover for an arid region? The Sandia National Laboratories in Albu-querque, N.M., may be able to help.

Currently, the laboratory is conducting a large-scale, alternative landfill cover experiment to identify less expensive and more environmentally-friendly techniques to cap hazardous waste landfills, particularly in arid regions. The project's results may directly benefit the nation's approximately 250,000 Resource Conservation and Recovery Act (RCRA) Subtitle D municipal landfills and its 6,000 Subtitle C hazardous waste landfills by offering cover alternatives.

According to the Los Alamos National Laboratory, installation of the U.S. Environmental Protection Agency (EPA)-approved RCRA cover design can cost more than $2 million per acre. Even worse, the EPA reportedly acknowledges that this cover is not entirely effective in arid regions.

The RCRA cover design, according to the EPA Design Guideline - Design and Construction of RCRA/ CERCLA Covers, uses a barrier layer composed of clay and a geo-membrane. Since the soil is compacted wet, this clay layer can dry and crack. Unfortunately, cracks provide possible pathways for water to pass through the soil and reach the landfill's hazardous materials. The resulting leachate poses a threat to groundwater reserves.

Another big problem is the lack of suitable clay near many western landfill sites. Often, a soil additive such as bentonite is mixed with native soil to create clay-like compaction characteristics. However, because the additive must be trucked in, this technique can be costly at large landfill sites and can result in negative environmental effects such as large bentonite excavations and heavy truck traffic.

Sandia's five-year cover project has culminated in a demonstration being conducted in two phases at landfill sites in dry mesa country, south of Albuquerque. Evaluation criteria include ease and cost of construction, performance and effectiveness.

Half of each 300-foot by 40-foot test site is being reviewed in its natural surrounding conditions. Sprinkler systems are located over the other half to simulate different precipitation conditions that place stress on the landfill covers. Sophisticated instruments, some developed specifically for the project, are used to measure the various covers' effectiveness in preventing water penetration.

In the first phase, three landfill covers are being tested: the RCRA standard Subtitle D municipal landfill cover; the RCRA-sanctioned Subtitle C hazardous waste cover and an alternative cover. The alternate cover is a manufactured geo-synthetic clay liner (GCL) that replaces the natural two-foot clay layer required by the RCRA guidelines. The GCL is engineered to block water far more effectively than a natural clay layer, according to the laboratory.

In the demonstration's second phase, several more innovative cover techniques are used such as: a capillary barrier method where water's capillary action impedes its migration from a higher, fine cover level to a lower, coarser level; an anisotropic barrier that encourages lateral water movement as opposed to downward; and an evapotranspiration cover that uses "engineered" vegetation to accelerate evaporation and plant transpiration to remove moisture from the soil (see chart on page 12).

Expected to be completed in 2000, the landfill demonstration should provide alternative covers with superior performance and long term reliability as compared with the RCRA design, while costing significantly less to install and maintain, according to the laboratory.

For more information, contact: Steve F. Dwyer, Sandia National Laboratories, P.O. Box 5800 MS-0719, Albuquerque, N.M. 87185. Phone/Fax: (505) 844-0543.