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Page i-17 SECTION t -LANDFILL EXPANSION ISSUES various locations,and the CIWMB will evaluate their adequacy. After such a ruling, the City could evaluate alternatives available based on performance and economic factors. 1.5.3 Final Cover The final cover for both the horizontal and vertical expansion phases will consist of a geomembrane cover system consisting of a foundation layer, a geomembrane, and a protective vegetation soil cover. As shown on the drawings the foundation layer is eighteen inches of compacted soil over the intermediate cover and an eighteen inch soil layer over the geomembrane. The depth requirements for these covers may change in the final design of the closures. In final design, the soil layer over the geomembrane may be subdivided into multiple layers including a drainage layer next to the geomembrane and a top soil layer for vegetation. As a point of clarification, there are essentially two final covers on the vertical expansion phases. The initial final cover will be placed within a few years and will consist of two feet foundation layer, one foot of compacted material with permeability less than 1 x 10$ cm/sec, and one foot protective vegetation layer. In twenty years, a liner will be placed on the vertical expansion area, waste will be placed, and then another final cover as described above will be constructed on top of the waste. Quantity requirements are discussed in the mass balance section below. Lo1:5.4 Uner Material The composite liner, for both the horizontal and vertical expansions,will be a two foot thick day liner with a maximum permeability of 1 x 10-' cm/sec. A soils investigation by Twining Laboratories in the area of Phase I and Phase 11 for the City of Stockton (dated October 24, 1991) showed that significant quantities of days appear to be available that could meet the permeability criteria. No permeability testing has been conducted on samples from the Phase III or Phase IV areas or the borrow area. EMCON boring logs in the borrow area show soils including clay, sandy day, sandy slit, clayey sand, silty sand, and sand. it is not possible at this time to determine if enough liner material is available on site for the liner construction of all the expansion phases. As site excavation proceeds, soil materials should be evaluated to determine if they meet the criteria for liners. Quantity requirements are discussed in the mass balance section mom below. 12 1.5.5 Site Soil Mass Balance The California Solid Waste Management Regulations require that the operator must be able to provide adequate material of suitable quality to meet the cover requirements. (Title 14, 17681) Also, availability of cover and liner soils has a significant impact on the operation cost and on the construction cost of liners and final covers. An overall mass balance of the site was conducted to determine if adequate soil quantities were available for operation and construction of liners and final covers. The daily, intermediate, and final cover soils for the existing landfill were included rim in the mass balance. The mass balance calculation included the following elements: ■ Liner soil, operation layer, and final cover for Phase i - VI R,W.eeac era AsscdaM City of sla&mn 1811=2 1 .