This site summarizes timely information about demonstrations and full-scale applications of alternative landfill covers. The alternative landfill covers described involve design concepts that primarily manipulate water balance principles to minimize percolation into the waste. Projects for this Web site are collected using information from technical journals and conference proceedings, as well as information obtained from technology vendors and site managers. The project profiles contain information about relevant site background, materials disposed of at the site, climate, location, monitoring system used, cover type, size and design, performance results, point of contacts, and references. This Web site can be used as a networking tool (each profile lists a contact) to identify past solutions and lessons learned that would apply to new sites with similar contaminants and climate.

The basic elements of A-ACAP's structure were decided at the November 2003 meeting of landfill operators, regulators and researchers that launched the project. WMAA and PhytoLink Australia organised this meeting because of the positive stakeholder response to the earlier WMAA/PhytoLink phytotechnology experts' seminar tour. In particular, participants expressed strong interest in evaluating phytocap performance under Australian conditions through a project that would take the approach and findings of the U.S. Alternative Covers Assessment Project as its starting point. A-ACAP is a large and complex project with a budget of $4 million that extends across 5 states and over 5 years.

In May 2004, lysimeters were constructed at Great River Energy's Coal Creek Station to allow a comparative field-scale evaluation of the environmental performance of three earthen final cover designs. Two of the cover profiles use conventional compacted clay hydraulic barriers to limit percolation, while the third profile is designed to store and release water via evapotranspiration. Although a considerable amount of progress has been made in modeling and understanding both types of cover systems in recent years, less attention has been devoted to the challenges introduced when attempting to obtain regulatory acceptance of an alternative final cover system. In this paper, the authors present a case history of the project, with an emphasis on permitting challenges associated with alternative final cover equivalency demonstrations. Cover system field performance to date is also discussed.

The protective cap/biobarrier experiment was constructed to compare the performance of three alternative evapotranspiration (ET) caps with that of a U.S. Resource Conservation and Recovery Act recommended cap. One alternative cap configuration was a homogenous soil cap, while two additional alternative cap configurations contained biointrusion barriers placed at different depths within a soil profile. All cap configurations were planted with two vegetation types and were tested under three climate-change precipitation scenarios. Under ambient precipitation, the three alternative cap configurations performed similarly and returned all of the water received from precipitation to the atmosphere during each growing season, regardless of vegetation type. Growing season ET on ambient-precipitation caps ranged from 133 to 338 mm. Native vegetation extracted more water from caps under augmented precipitation regimes than did a grass monoculture. End-of-season soil volumetric water content was 1.7�5.9% lower on caps planted with a native species mix. When planted with native vegetation, the alternative cap designs tested here should easily meet United States Environmental Protection Agency equivalency criteria.

This study focuses on evaluation of evapotranspiration defined using field monitoring of water balance components and estimated using empirical models. Even though significant emphasis was placed on vegetation development at the site, assessment of site monitoring results indicates that evaporation from the cover surface removes 1.5 times more water than plant transpiration. Overall, evaporation was sufficient to elicit satisfactory cover performance.

U.S. EPA Subtitle D municipal solid waste landfill requirements specify that the permeability of a cap to a landfill be no greater than the permeability of the underliner. In recent years the concept of the evapotranspirative (ET) cap has been developed in which the cap is designed to store all rain infiltration and re-evapotranspire it during dry weather. Concern at the long period required for landfilled municipal solid waste to decompose and stabilize in arid and semi-arid climates has led to an extension of the concept of the ET cap. With the infiltrate-stabilize-evapotranspire (ISE) cap, rain infiltration during wet weather is permitted to enter the underlying waste, thus accelerating the decomposition and stabilization process. Excess infiltration is then removed from both waste and cap by evaporation during dry weather. The paper describes the construction and operation of two sets of experimental ISE caps, one in a winter rainfall semi-arid climate, and the other in a summer rainfall semi-arid climate. Observation of the rainfall, soil evaporation and amount of water stored in the caps has allowed water balances to be constructed for caps of various thicknesses. These observations show that the ISE concept is viable. In the limit, when there is insufficient rainfall to infiltrate the waste, an ISE cap operates as an ET cap.

USDA scientists have worked with EPA and private consultants to design and conduct a pilot study of an alternative way to cap landfills. The pilot project is being conducted in Maryland on part of a long-abandoned, 30-acre municipal landfill within the 6,615-acre Beltsville Agricultural Research Center complex. The idea is to cap or seal the old landfill with trees and shrubs planted in a mix of topsoil and compost, instead of the traditional clay cap. Vegetative caps reduce methane emissions while preventing rainfall from penetrating into the municipal waste and then leaching into groundwater. Also, an increase in forest canopy contributes to improving the health of local water bodies by sequestering carbon and filtering runoff. If accepted by the State of Maryland, the approach when fully implemented would create more than 30 acres of forest canopy and critical habitat. The project is illustrated in the poster, "An Innovative Approach to Landfill Capping: A Joint Environmental Unit & Research Project at the College Park Landfill."

Results are presented here of a field study to evaluate the relative hydrologic performance of various landfill cover technologies installed at Hill Air Force Base, Utah. Four cover designs (two capillary barrier designs, one modified EPA RCRA design, and one control cover) were installed in large lysimeters instrumented to monitor the fate of natural precipitation between January 1, 1990 and September 20, 1993. After 45 months of study, results showed that the cover designs containing barrier layers were effective in reducing deep percolation as compared to the control cover. The RCRA cover, incorporating a clay hydraulic barrier, was the most effective of all cover designs in controlling deep percolation but was not 100 percent effective. The two capillary barriers were successful in reducing deep percolation, but significant amounts were still produced. Over 90 percent of all percolation through the covers and lateral flow within the covers occurred during the months of February through May of each year, primarily as a result of snowmelt, early spring rains, and low evapotranspiration. Gravel mulch surface treatments (70 to 80 percent ground cover) were effective in reducing runoff and erosion. The two plots receiving the gravel mulch treatments exhibited equal but enhanced amounts of evapotranspiration, despite the fact that one plot was planted with additional shrubs.

Since 1986, different types of landfill covers have been studied in situ on the Georgswerder landfill in Hamburg, Germany. Water balance data are available for eight years. The performance of different barriers has been measured by collecting the leakage on areas ranging from 100 m2 to 500 m2. Composite liners with geomembranes performed best, showing no leakage. An extended capillary barrier also performed well. The performance of compacted soil liners, however, decreased severely within five years due to desiccation, shrinkage and plant root penetration (liner leakage now ranging from 150 mm/a to 200 mm/a). About 50% of the water that reaches the surface of the liner is leaking through it. The maximum leakage rates have increased from 2 x 10-10 m3 m -2 s-1 to 4 x 10-8 m3 m-2 s -1. Two types of geosynthetic clay liners (GCL) have been tested for two years now with disappointing results. The GCL desiccated during the first dry summer of the study. High percolation rates through the GCL were measured during the following winter (45 mm to 63 mm in four months). Wetting of the GCL did not significantly reduce the percolation rates.

This project addresses a need for confirming methods of keeping water from waste. The concepts under investigation are applicable to near surface facilities as well as mined caverns. The project is significant in that it presents results of 12 years of actual cover performance at a humid region site. Long-term field projects on this scale are rare because of cost. Consequently, most reports on cover performance appearing in the literature are computer simulations. Of the concepts under investigation, two are particularly promising and are unique to this project. The first is a surface cover called bioengineering. Because it is a surface cover it is easy to maintain should there be subsidence. Bioengineering has the capability of (1) reducing infiltration of water through a cover to zero and (2) the remedial action capability of lowering the water table beneath a cover. The latter capability is a very important property for cleaning up sites in which there is water in disposal units. The second promising concept is called a conductive layer barrier. This is a special application of a capillary barrier, in which a capillary break is placed below a conductive layer. The conductive layer consists of material (e.g. fine sandy loam) which is capable of conducting water around waste under unsaturated flow conditions. In the absence of subsidence, such a system offers a significant margin of safety to cover performance particularly when it is placed below a geomembrane or a clay layer or a GCL, and it has a wide range of possible applications ranging from a tumulus to mined cavern disposal where there is intermittent fracture flow of water.

Two instrumented test sections were constructed in summer 1999 at the Kiefer Landfill near Sacramento, CA, to test the hydraulic performance of two proposed alternative final covers. Both test sections simulated monolithic ET designs that differed primarily in thickness. Both were seeded with a mix of two perennial and one annual grass species, and oleander seedlings were planted in the thicker test section. Detailed hydrologic performance monitoring of the covers was conducted from 1999 through 2005. Decommissioning showed that the properties of the soils changed over the monitoring period and the perennial grasses and shrubs intended for the cover were out-competed by annual species with shallower roots and lesser capacity for water uptake. Results underscore the importance of establishing and maintaining appropriate vegetation on ET covers in this climate.