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3.3. WASTE DECOMPOSITION PROCESS AND WASTE BY-PRODUCTS <br /> Municipal solid wastes placed in the Foothill landfill will undergo natural chemical and <br /> biological decomposition following disposal. The products of biological decomposition of <br /> organic wastes are solids, liquids, and gases. <br /> The waste decomposition process begins with organic waste products undergoing aerobic <br /> (presence of free oxygen) decomposition during storage and transport, and for a limited period <br /> after placement in the landfill. Aerobic decomposition continues until available oxygen is <br /> depleted. When oxygen is depleted, anaerobic (lack of oxygen) decomposition becomes <br /> dominant. <br /> Typical byproducts of aerobic decomposition of MSW are carbon dioxide, water, and nitrate. <br /> Typical primary products of anaerobic decomposition are methane, carbon dioxide, organic <br /> acids, nitrogen, ammonia, iron sulfides, manganese, and hydrogen. These products are the <br /> primary factors controlling the quality and quantity of leachate and landfill gas produced. <br /> Methane in the gas may range from 50% to 60% and carbon dioxide may range from 30% to <br /> 50% (Boyle, 1977). <br /> Methane production from MSW will continue from a few years to hundreds of years, in certain <br /> environments. The rate of gas production depends on a number of parameters: refuse <br /> composition, oxygen present, moisture content, pH, alkalinity, and temperature. <br /> 3.3.1. Subsurface Soil Gas Monitoring <br /> Twelve soil gas monitoring wells (SG-1 through SG-12) are installed to allow monitoring the <br /> characteristics of the gas in the soil near the landfill (Appendix P). Well SG-12 is provided with <br /> three sampling probes while the others all have one probe each. Well depths vary from 25 to 68 <br /> feet deep, such that the elevation of the bottom of the sampling section at the elevations of the <br /> refuse anticipated in the future within 1000 feet of the well. In addition, a soil gas trench is <br /> installed beneath the liner in Module 1. These features allow monitoring for subsurface of <br /> landfill gas (LFG) across the property boundary, and monitoring for subsurface VOCs. <br /> Foothill Sanitary Landfill Page 12 of 62 Public Works/Solid Waste <br /> JTD County of San Joaquin—Revised 10/6/05 <br />