For 26 years, Orange County, Calif., operated the Santiago Canyon Landfill before it closed in 1993. Built without an underlayer to prevent leachate from entering the groundwater, the facility was redesigned to meet state and federal closure regulations.
The site's cramped conditions prompted several changes in the preliminary closure plan. "The site had been graded to the edge of the property," said Carl Nelson, director of public works programs at Holmes & Narver, the engineering firm that designed the final landfill improvements. "There wasn't much room left for working."
The original plan called for collecting drainage with concrete-lined peripheral drains installed along the east and west perimeters of the landfill footprint. Downdrains and interceptor benches on the landfill face would drain into the peripheral channels, which would then empty into a single desilting basin at the north end of the canyon near the property boundary.
Current regulations require on-site retention of the volume of sediment anticipated from a 100-year, 24-hour storm, according to Nelson. "Accepted practice requires a trap settling volume of 67 cubic yards per acre of watershed. Our analysis showed that the proposed single basin could not accommodate that volume without infringing on the landfill footprint," he said.
To combat this problem, the new design provides two separate, concrete-lined basins on different areas of the site with a total volume of 13.5-acre feet - significantly larger than the original basin - which allow optimum sediment settling.
Since there was no room along the property boundary for standard, concrete-lined drainage ditches, the new design also created a flexible drainage system across the landfill face using wire-wrapped rock (gabion) channels underlain by a flexible membrane liner.
Since it was impossible to provide a reasonable grade for access roads along the sloped perimeter, the final design uses a switchback, or serpentine, road configuration across the landfill face to avoid grades greater than 10 percent. The design provides all-weather access from the entry station to the landfill gas flare station and to the top of the landfill, where cover material is delivered for final closure. Since the landfill is currently open to commercial haulers for disposing limited amounts of clean earth, the roads also provide temporary access to stockpile areas for ongoing operations.
Creating access roads required complicated design modifications. In addition to providing adequate support for pavement over the settling landfill, the road base had to accommodate the eventual installation of a final landfill cover composed of a layer of low-density polyethelene sandwiched inside two feet of earth in place of the usual clay cap. The road design incorporated the same liner beneath the road prism, which could later overlap and bond to the cover liner.
To improve stability along the western ridge of the landfill, the geotechnical engineers recommended that the over-steep and cut-slopes cover the face with a compacted buttress fill. The design extended the gas pipes to the interior edge of the buttress fill to keep the gas system accessible to maintenance workers.
After closure, leachate will be regularly collected and disposed. Downslope monitoring wells will be regularly checked to ensure that leachate does not escape into the groundwater. The site's semi-arid climate and source material should produce minimal amounts of leachate.
"In regions with year-round rainfall, a similar landfill closure plan usually would have to contend with much larger amounts of downstream leachate, making collection and disposal a more costly proposition," said Nelson.
Finally, as part of the stormwater pollution prevention plan, automatic samplers in each retention basin will retrieve surface water samples for the Regional Water Quality Control Board.