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<br /> <br />Foothill Sanitary Landfill Page 44 of 52 San Joaquin County DPW - Solid Waste <br />Joint Technical Document Revised 01/05/2026 <br /> <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 /> <br />The waste decomposition process begins with organic waste products undergoing <br />aerobic (presence of free oxygen) decomposition during storage and transport, and for a limited <br />period 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 /> <br />Typical byproducts of aerobic decomposition of MSW are carbon dioxide, water, and <br />nitrate. Typical primary products of anaerobic decomposition are methane, carbon dioxide, <br />organic acids, nitrogen, ammonia, iron sulfides, manganese, and hydrogen. These products are <br />the 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 /> <br />Methane production from MSW will continue from a few years to hundreds of years, in <br />certain environments. The rate of gas production depends on a number of parameters: refuse <br />composition, oxygen present, moisture content, pH, alkalinity, and temperature. <br /> <br />LFG Surface emissions are monitored by a “surface scan” test, in which the methane <br />content of the air immediately above the landfill is measure in accordance with EPA Title V <br />Regulations and the Permit to Operate (Appendix A). This measurement indicates that surface <br />emissions are within the regulatory limits. <br /> <br />Surface emissions are controlled by the LFG collection and treatment system (Figure <br />15). This system consists of thirty-four vertical wells and eighteen horizontal collectors, each of <br />which is provided with an adjustment valve, sampling port, and temperature gauge. The LFG <br />collection rate at each well is adjusted based on the methane content at each well. The vacuum <br />is increased or decreased to maintain the methane content within a prescribed range. <br />