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The Pit 7 complex is underlain by a perched water bearing zone located at the lower portion of the Neroly formation at depths of about 20 to 40 feet beneath the surface. The most common apparent saturated thickness within the perched aquifer is six to eight feet. Pit 1 area is underlain by an extensive semi -confined aquifer about 120 feet below the surface. The aquifer's thickness varies. There are other deeper water bearing zones and they may be interconnected. The general direction of groundwater flow beneath Pits 1 and 7 is northeastward. No natural perennial surface water bodies exist at Site 300. Rainfall flows through channels, drainage ditches and ponds where there are depressions until it soaks into the soil. All water requirements at Site 300 comes from the groundwater. Water for the entire site is provided by two wells in the GSA area about four miles from the Pit 7 complex area. There are no known uses of the shallow perched groundwater within the Pit 7 complex area because the water yield of wells around this complex is so low, and natural water quality is poor. IV. OVERVIEW OF PITS 1 AND 7 CLOSURE PLAN The closure activities will be performed by contractors and subcontractors to LLNL. The closure activities will be implemented, after final approval of the closure plan, during the months of April to September. The cover design includes a uniform surface slope of at least 3 percent and up to 5 percent. This design will promote drainage of rainwater run-off to the two drainage ditches located on each side of the landfill, most of which will finally drain to Elk Ravine. Good drainage design and a thick clay barrier will prevent infiltration of rainwater into the landfill and will minimize release of hazardous constituents. The following describes each component of the cover design of Pits 1 and 7 starting from the bottom layer (shown in Figure 3): a. Temporary soil cover - Pits 1 and 7 are currently covered with six inches to five feet of clayey sand and sandy silt. Both soils are derived from on-site borrow areas. This layer will remain as part of the final cover. b. Foundation layer - This one -foot layer of soil is made up of the same type of soil as the temporary cover. c. Law permeability layer - this consists of two -foot thick layer of imported high-grade kaolinitic clay. When compacted, this kind of clay has a hydraulic conductivity of 1 x 10-7 cm per second (0.023 inches per day) or lower meaning water will soak in at this rate if not drawn off by the drainage layer. This type of soil will minimize liquid soaking into the waste. d. Drainage layer - this consists of one -foot layer of imported sand/gravel mixture. The purpose of this layer is to facilitate the efficient removal of water to an exit drain.