From ALCD to ASTD
An Alternative Landfill Cover Demonstration taking place at Sandia National Laboratory is providing data by which to evaluate the cost and performance of six differently constructed landfill covers. A favorable showing by one of the covers indicates that it’s both an effective and low-cost option for stemming the flow of water to lower soil layers—an important consideration since water infiltrating a landfill raises the risk that landfill contaminants will leach into surrounding soil and groundwater. Through the data being generated by the ALCD project, DOE is gaining regulatory approval for deploying alternative cover designs that perform as well as and are less costly to construct than baseline Resource Conservation and Recovery Act–approved covers, which are particularly ineffective and costly in arid climates. The Office of Science and Technology’s Subsurface Contaminants Focus Area is funding the alternative landfill cover side-by-side demonstration (see Initiatives, December 1995). Two years of monitoring data have been collected thus far; a minimum five-year study is planned.

During the first year of collecting data from the ALCD (May ’97 to March ’98), Principal Investigator Steve Dwyer found that an evapotranspiration (ET) soil cover was a top performer in this competition that pits four alternative landfill cover designs against two RCRA–approved covers. On the basis of the ET cover’s early performance in the ALCD, Dwyer proposed in 1998 that the ET cover be deployed under OST’s Accelerated Site Technology Deployment (ASTD) program for capping a 2.6-acre mixed waste landfill at Sandia National Laboratories. The deployment of the evapotranspiration cover was subsequently selected by OST as one of fourteen 1998 ASTD projects.

Why an alternative cover?
Experience in the Western United States has shown that EPA-recommended landfill cover designs for facilities regulated by RCRA Subtitles C and D are unsuitable in arid regions. These standard RCRA covers’ barrier layers are composed of natural clay, which dries and cracks in areas where rainfall is scant. In addition, these covers are especially expensive in the West, where clay is not readily available. To comply with RCRA guidelines, many site operators must truck in a soil additive such as bentonite and mix it with native soil to create clay-like compaction characteristics. This process can be a massive and costly undertaking. According to estimates developed at Los Alamos National Laboratory, the cost of installing a RCRA-approved cover can reach more than $2 million per acre in arid regions.

But other parts of the country are having problems with their landfills as well. An EPA study of randomly selected landfills revealed that the vast majority are leaking, which contributes to groundwater contamination and other environmental problems. The California Solid Waste Assessment Test Report found that 72 to 86 percent of existing landfills with compacted clay barrier layers are failing all over the United States.

How is an ET cover made?
The ET cover consists of a compacted soil barrier layer with an engineered vegetative cover to facilitate water storage and evaporation of moisture into the atmosphere through the process of plants’ transpiration. As constructed at the ALCD, the ET cover is 90-cm-thick—a 75-cm bottom layer overlain with 15-cm topsoil loosely placed. An inch-thick layer of gravel on the surface reduces surface erosion and helps plants establish themselves.

The secret to the ET cover’s success appears to be its depth. While the RCRA Subtitle D soil cover (to meet EPA standards for municipal solid waste landfills) is constructed similarly to the ET cover, the RCRA cover is a poor performer due to its depth—a 45-cm-thick compacted soil barrier layer topped with 15-cm topsoil for vegetation. Computer modeling has revealed that if the depth were increased to 90 cm, the RCRA Subtitle D cover would be more successful at preventing water infiltration into underlying waste.

The proof’s in the data
Automated monitoring systems are collecting water balance data for each cover installed at the ALCD. Water balance variables include surface runoff, lateral drainage, soil water storage, and percolation. The covers are judged on the basis of comparable percolation rates—a direct comparison of the covers’ impermeability. A final cap on a landfill must be able to isolate the underlying waste for the length of time it takes for the waste to be deemed harmless to the surrounding community.

Tracking the performance of the ET cover
During the first year of this ASTD project, an ET cover installation was designed. In FY99, the project’s design, along with supporting data from the ALCD, will be submitted to New Mexico regulators for approval, and the cover will be installed. The project will finish next year with the collection of the ET cover’s first year of monitoring data.

After the ET cover is installed, the project will focus on providing data that confirm that the cover is, indeed, repelling water. The Fiber Optic Fluid Infiltration Monitoring System will be installed between the ET cover’s layers to monitor water conditions. This system is based on the observation that as the amount of water in soil increases, there is a corresponding increase in thermal conductivity, which is a measure of how quickly heat dissipates from its source into surrounding soil layers. The presence of water will cause a reduction in the temperature of the soil immediately around a heating element as thermal energy is drawn from the immediate area into areas of the landfill more distant from the source of heat.

An electrical wire will generate a constant source of heat into the landfill. When water is present, the fiber optic system will measure a relative reduction in temperature of material at the heating element as thermal energy is conducted away from the heat source. Calibration is suggested to be strictly empirical, since the thermal properties of materials and interfaces will be site-dependent.

YORK in the United Kingdom developed this commercially available optical fiber monitoring system, which is used by Pruett and other companies in the United States in industrial, oil field, and geothermal applications. The system incorporates both intrinsic and distributed elements: sensors measure environmental effects on the fiber itself, while continuous lengths of fiber read the propagation delays of light traveling along the fiber relative to distance. Measurement accuracy is +1°C with resolution of about 1 m over lengths up to 10 km. The system’s Raman distributed temperature sensors are now well established and practical with applications to electrical machines, cables and transformers, location of fires, and industrial plants.

The optical fiber sensor and the heating wire are bundled together in stainless steel or Teflon. These bundled cables can be placed horizontally between landfill layers. The cost is less than $5 per meter for the stainless steel cable. This method, measuring the ratio of Stokes to anti-Stokes backscattering of light in a fiber, provides an absolute indication of the temperature of the surrounding medium regardless of light intensity, launch conditions, fiber geometry, and materials.

Promoting alternative covers
Stephen Dwyer, who is the principal investigator on both the ALCD and the ASTD cover project, is busy these days working with New Mexico regulators to gain approval of the ASTD project, compiling data from the ALCD, and promoting alternative covers to close out DOE landfills across the country. Dwyer says that DOE has an estimated 3,000 landfill sites under its management and is interested in identifying more cost-effective and environmentally friendly ways to seal off each site as it reaches the end of its useful life.

“As more data become available from the ALCD, deployments of alternative covers will become more widespread. Currently, deployments include two landfills at Sandia, one site in Idaho, and one in Wyoming. Many site personnel and regulators from several states have toured the ALCD for information to help them with landfills that will soon be closing at their respective sites.”

For more information about alternative landfill covers, contact Steve Dwyer at (505) 844-0595, sfdwyer@sandia.gov .