The United States Environmental Protection Agency’s (USEPA) Alternative Cover Assessment Program (ACAP) is being conducted to characterize the performance of alternative landfill cover designs employing water balance principles in a variety of climatic conditions. There is growing interest in alternative landfill cover designs because these covers are perceived to be less costly to construct and maintain than conventional landfill covers, designed as resistive barriers, while providing comparable long-term protection of the environment.

Under this study, test facilities were constructed at ten sites across the United States to assess the hydrological performance of alternative and conventional covers. Conventional and alternative cover designs are being evaluated in a side-by-side comparison at seven of these sites. Each cover was constructed with locally available soils and vegetated with local grasses, shrubs, and forbs. The water balance of each cover was monitored using the large (10 m x 20 m) pan-type lysimeter. The lysimeter is constructed with geomembrane and contains a geocomposite drainage layer along the base to collect percolation. Surface runoff is collected by diversion berms. Percolation and runoff are collected in 100-L basins equipped with a dosing siphon for automated drainage (Benson et al. 2001). The volume of water collected in each basin is measured using a pressure transducer and drainage of the basins is recorded by a float switch. Flow in the percolation basin is also measured with a tipping bucket. Meteorological conditions were measured with an on-site weather station. Disturbed and undisturbed samples were collected during construction from four quadrants in each lift of the test sections for measurements of physical and hydraulic properties. Field samples of vegetation were collected from the test sections periodically for measurement of leaf area index and root length density. Site maintenance visits were made annually to conduct quality control checks and to calibrate instruments.

Hydrological performance data, as well as meteorological data, are being continuously monitored for a range of 2-6 years. Soil hydraulic properties and plant properties are also measured from samples collected during annual site visits. The objective is to collect a sufficient amount of field data to assess the performance of the alternative water balance cover designs. The study has three specific objectives: (i) to determine effective climatic locations for water balance covers; (ii) to determine effective equivalency criteria and monitoring periods for water balance covers, and (iii) to provide an effective design method for water balance covers.