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Page 40 V A base liner would be installed,with the purpose of containing leachate within the landfill so that it can be channeled and collected. For the horizontal expansion,the base liner would be placed upon the prepared excavation at the site. For the vertical expansion,the base liner would be placed upon the final cover of the existing landfill. From top to bottom,the typical base liner design includes an 18-inch operation/drainage layer,a 60 mil high density polyethylene(HDPE)liner,and a 24-inch compacted clay - layer having permeability of 1 x 10 cm/sec. The HDPE liner would serve both as an added barrier to protect groundwater and as a flow surface for leachate. Groundwater varies between-48 feet to-12 feet mean sea level(MSL),and the maximum depth of the liner,not including leachate collection trenches and sumps, is 20 feet MSL,resulting in a minimum separation of 32 feet between the waste and groundwater. Sludge Treatment } No sludge treatment is included in the conceptual design for the Austin Road Landfill expansion. At the time the 1993 Draft EIR was written,the City of Stockton Municipal Utility Department had stockpiled digested sewage sludge at the landfill. This sludge has subsequently been removed(Griottonini,pers. com.). Leachate Coliection Leachate is any liquid that has come in contact with or percolated through refuse. The proposed landfill's leachate collection plan includes a drainage system,a leachate collection system,and a leak detection system. The drainage system would consist of a drainage layer and an operation layer. The drainage layer would -1 consist of either a minimm uof 12 inches of sand or gravel with a minimum permeability of 1 x 10 cm/sec or a geonet drainage layer with similar performance. The operation layer would be above and protecting the drainage layer. The operation layer would be 12 to 18 inches thick,depending on the composition of the drainage layer,and separated from the drainage layer by a geotextile filter. A leachate collection and removal system(LCRS),consisting of leachate collection trenches,collection pipes,and collection sumps, is required for all areas of landfill footprint. The LCRS would be integrated into the base liner by forming a network of trenches in the base liner,and placing the collection pipes in liner. In the leachate collection trenches the perforated collection pipes would those trenches above the pe be surrounded by drainage gravel, acting as a secondary leachate collection area if a pipe should fail. The drainage gravel around the collection pipe would be wrapped in a geotextile filter to keep it separate from the drainage layer. The leachate collection trenches and pipes would feed into collection sumps located at the low points of the landfill liner system. The leachate sump includes a geomembrane sump basin to allow submergence }� of the leachate discharge pump and free drainage of the collection trenches. A series of leachate pumps is proposed for the Austin Road Landfill expansion,because of the minimal topographic relief at the.site, the liner depth limit of 20 feet MSL,and the extensive area covered by the landfill. This would provide for hydraulic isolation of specific landfill areas for stormwater.managcment of a failed sump basin. The leachate collection pumps would discharge to a perimeter leachate header force main. The main would transport the leachate to the leachate collection holding facility,northwest of the existing landfill. Pumper trucks would collect the leachate and haul it to a licensed Treatment and Disposal Facility for treatment. If necessitated by the quality of the leachate,a pretreatment program would be defined. A leak detection system would consist of a series of secondary collection trenches located beneath the GRASSETTI ENVIRONMENTAL CONSULTING