Field Hydrology of Landfill Final Covers with Composite Barrier Layers

A study was conducted at seven sites across the United States to evaluate the field hydrology of final covers with a composite barrier (a geomembrane over a soil barrier or a geosynthetic clay liner) for final closure of landfills. The water balance of each cover was monitored with a large ($10\times 20 \text{m}$) instrumented drainage lysimeter. With one exception, the covers limited the average annual percolation to < 2.8 mm/year (< 0.4% of precipitation). The geomembrane barrier at one site (Marina, California) was likely damaged during construction; percolation at this site averaged 30 mm/year (6.9% of precipitation). The annual percolation through the cover at the wettest site (Cedar Rapids, Iowa) ranged between 0.1 and 6.2 mm/year. The annual percolation at arid and semiarid sites was typically no more than a trace (< 0.1 mm/year). Percolation from all test covers generally was coincident with high water storage in the surface soil layer and lateral flow in the drainage layer on the surface of the geomembrane barrier. Water balance predictions were made with the hydrologic evaluation of landfill performance model using site-specific input. Surface runoff was overpredicted and evapotranspiration underpredicted when as-built soil hydraulic properties were used as input. Better agreement was obtained when in-service soil hydraulic properties were used as input. The lateral flow was consistently overpredicted regardless of the hydraulic properties, and no correspondence existed between the predicted and measured percolations.