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II. Project Descri tion J P 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. It is recommended by the design engineers that the geomembrane sump basin be prefabricated and thoroughly tested for leakage prior to installation because of the potential problems due to the leachate pressure buildup required for pump efficiency. Field seams should be above the high leachate pump operation level. A series of leachate sumps as opposed to one sump are proposed for the Austin Road Landfill expansion. The reason is 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. Several sumps result in higher operation and maintenance costs;however,they do provide for hydraulic isolation of specific landfill area for stormwater management or 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 the Stockton Municipal Utility District for treatment. Leachate at the holding facility would be removed as necessary. Leak Detection System The leak detection system would consist of a series of secondary collection trenches located beneath the liner and directly below the leachate collection trenches,providing a secondary containment beneath the leachate collection trenches and sumps. The secondary collection trenches are lined with a geomembrane liner and use a drainage gravel and collection pipe similar to the leachate collection system. This leak detection system would act as a vadose monitoring system to the degree that it can monitor for leakage through the liner. It would not however,be representative of the soil-pore liquid quality in the unsaturated zone beneath the water table. A standard vadose monitoring method would not be practical at this landfill because of the extensive area and depth of the developed landfill. I1.13