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II. Project Description directed towards the expansion phases(please refer to Impact K.3.for a further description). In addition,an intermediate perimeter ditch would be provided to collect run-off from the existing j landfill and to prevent run-on toward active landfilling areas. Design of the perimeter ditches would provide for capacity of the 100-year,24-hour storm. Run-off from areas with intermediate closure would be directed to the perimeter surface water ditches,provided that the tun-off is not contaminated by leachate. Side slopes would be carefully monitored for leachate seepage. Silt fences would be used as necessary in active and intermediate landfill closure areas to control soil erosion. Once the proposed landfill is closed,surface water would be collected by a network of interception ditches on the slope. Interception ditches would have an appropriate lining based on design velocities. The interception ditches would feed into catch basins connected to the perimeter ditch system. The perimeter ditch system would take the water to the detention facility. Culverts in the catch basins would be designed to include outlet control structures to protect the perimeter ditch lining. This system is designed to handle a 100-year,24-hour storm. The surface water detention facility would be located in the southwest corner of the site. The design and construction of the facility would meet the requirements of the San Joaquin County 1 design standards and sized to handle a 100-year,24-hour storm incident on the closure area of both the existing and expanded areas. The detention pond would be excavated below the current existing grade and pump its discharge to the North Branch of the South Fork of Little Johns Creek. The pump would be designed to empty the detention basin within 48 hours. The detention basin facility would be fenced to prevent unauthorized access. Landfill Gas Control When organic waste is placed in a landfill,it initially contains oxygen,and decomposes aerobically in the presence of oxygen for a short time,producing only carbon dioxide. Anaerobic decomposition,which takes place after the oxygen is depleted,produces both methane and carbon dioxide. The two primary concerns over landfill gas production are that landfill gas is usually odorous and that methane can be present at concentrations where it would be flammable or explosive. While neither carbon dioxide nor urethane are odorous,some trace landfill gasses are odorous(including hydrogen sulfide). Landfill gas is usually saturated with water vapor and is typically at an elevated temperature due to the decomposing activities in the 11 landfill. Thus,when the landfill is collected in pipelines a liquid condensate is generated. I 11.16