Alternative final covers for landfills in arid and semi-arid regions - A case study

The Coyote Canyon Landfill is an approximately 325-acre California municipal solid waste facility which was operated by the Orange County Integrated Waste Management Department between 1963 and 1991. Between 1992 and 1994 the disposal area was closed in accordance with then current minimum regulatory standards as defined in Titles 14 and 23 of the Code of California Regulation (CCR). These regulations are generally consistent with the now current closure requirements of 40 CFR (Subtitle D) for unlined landfills. The final closure plan for the landfill was modified in 1993 to allow for the development of a threatened species (California Gnatcatcher) habitat over an approximately 100 acre portion of the site. While still incorporating a prescriptive low permeability soil layer at depth, the Gnatcatcher habitat revisions necessitated the construction of a thickened vegetative soil layer to accommodate the coastal sage scrub (Gnatcatcher habitat) plant community. Owing to the relatively deep root distribution of Coastal Sage Scrub, the California regulatory agencies mandated that the moisture content in the vegetative and barrier layer soils be monitored in order to evaluate their long term performance characteristics. While perhaps not the original intent of the agencies, the data recovered from the monitoring systems installed at the Coyote Canyon Landfill clearly indicate that the performance of the system in this semi-arid climate is governed by evapotranspiration within the vegetative zone. Furthermore, the data also indicate that the prescribed low permeability soil layer does not provide significant protection against water infiltration into the refuse prism. The conclusion that the unsaturated barrier-layer at the Coyote Canyon Landfill provides little, if any, protection against infiltration suggests that current regulatory requirements to construct similar soil barrier layer systems at other landfills in arid and semi-arid regions may be unnecessary and, if constructed, may provide little in the way of true environmental protection.