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LANDFILL PRODUCTS: GENERATION AND MIGRATION <br /> <br />GAS <br /> <br /> The decomposition of organic matter in a landfill occurs <br /> <br />in several stages. Initially, some of the ·organic matter is de- <br />composed aerobically; that is, in the presence of oxygen. <br />Decomposition products at this stage include carbon dioxide, water, <br />organic compounds, and heat. Combustible gases are not formed <br />at this point. <br /> <br /> Methane generation begins when anerobic (without oxygen) <br />conditions are established. During this stage the waste material <br />is reduced to organic acids and alcohols by acid-forming bacteria. <br />Finally, methane-forming bacteria reduce the acids and alcohols <br />to methane. <br /> <br /> The quantity and quality of gases produced depends on <br /> <br />several factors including: temperature, pH, moisture content, <br />nutrient availability, microogranism distribution, landfill age <br />and composition, and oxygen content. <br /> <br /> Gas production may continue for 60 to 80 years, and the peak <br />methane production period is reported-to be 15 to 20 years after. <br />establishment.of the site (Zimmerman and ~ilkey, 1979). · <br /> <br /> Gases generated within the landfill wilt travel in the <br />direction of least resistance. Lateral migration of-gases has ~ <br />been reported to be a problem in landfills built on former <br />sand and gravel pits. Gas migration will depend on characteristics <br />of surrounding soils, soil moisture, the distance to the water <br />table, the depth of the landfill, gas generation, surface runoff, <br />and the presence or absence 'of natural barriers to soil venting <br />such as clay layers or frozen ground. Lateral migration of gases <br />in the soil at distances up to 1,000 feet has been reported <br />(Flower et al., 1979).. <br /> <br /> Carbon dioxide and methane are the major landfill decomposition <br />products. Minor fractions may include hydrogen sulfide, nitrogen, <br />oxygen, carbon monoxide, paraffin and cyclic hydrocarbons, and <br />volatile organic acids (Ham et al., 1977. IN: Zimmerman and Wilkey, <br />1979). Table 1 shows the percent composition of gases from a <br />sanitary landfill reported by Ham et al., 1977. <br /> <br /> <br />