【正文】 hich is consumed by sulfate reducers (Kasali, 1986). Qualitatively, land?ll gas is highly dependent on the deposition stage within the land?ll (Rovers and Farquhar, 1973。 Rees, 1980。 Pohland et al., 1983。 Barlaz et al.,1989c). Under a stabilized methanogenic condition which is the stage of interest from a bene?cial recovery perspective, methane and carbon dioxide are by far the two principal ponents of land?ll gas and form more than 90% of the total gas and oxygen are normally present in small quantities primarily as a result of air entrapment during waste deposition, atmospheric air di?usion through the land?ll cover especially in the near surface layers, or air intrusion from negative land?ll pressure when land?ll gas is extracted. 3. Leachate Formation Mechanisms Leachate formation is the result of the removal of soluble pounds by the nonuniform and intermittent percolation of water through the refuse mass. Soluble pounds aregenerally encountered in the refuse at emplacement or are formed in chemical and biological processes. The sources of percolating water are primarily the precipitation,irrigation, and run o? which cause in?ltration through the land?ll cover。 ground water intrusion, and to a lesser extent, the initial refusemoisture deposition due to microbial activity may also contribute to leachate formation but in smaller amounts. The quantity of leachate generated is sitespeci?c and a function of water availability and weather conditions as well as the characteristics of the refuse, the land?ll surface, and underlying soil. The quality of land?ll leachate is highly dependent upon the stage of fermentation in the land?ll, waste position, operational procedures, and codisposal of industrial wastes (Hoeks and Harmsen, 1980。 Parker and Williams, 1981。 Harmen, 1983。 Pohland et al., 1983). Many chemicals (. metals, aliphatics, acyclics, terpenes, and aromatics) have been detected in land?ll leachate from domestic, mercial, industrial, and codisposal sites. 4. Environmental Impacts Historically, land?lls were initiated largely as a result of a need to protect the environment and society from adverse impacts of alternative methods of refuse disposal such as openair burning, openpit dumping, and ocean dumping (Senior, 1990). Although land?lls eliminated some impacts of old practices, new ones arose, primarily due to gas and leachate formation. Besides potential health hazards, these concerns include ?res and explosions, vegetation damage, unpleasant odors, land?ll settlement, ground water pollution, air pollution and global warming. Fire and Explosion Hazards Although land?ll gas rich in methane provides an energy recovery opportunity, it has often been considered to be a liability because of its ?ammability, its ability to form explosive mixtures with air, and its tendency to migrate away from the land?ll boundaries by di?usion and advection. Di?usion is the physical process that causes a gas to seek a uniform concentration throughout the land?ll volume, hence the gas moves from areas of higher to areas of lower concentration. Advection results from pressure gradients where gas moves from zones of higher to zones of lower pressure. Di?usion and advection rates depend primarily on the physical properties and generation rates of the land?ll gas, refuse permeability, internal land?ll temperature, moisture content surrounding soil formation and changes in barometric pressure. Damage At closure, many land?ll sites are converted to parks, golf courses, agricultural ?elds, and in some cases, mercial developments. Vegetation damage at or nearby to such sites is well documented in the literature (Flower et al., 1977, 1981。 Leone et al., 1977。 Leone and Flower, 1982。 Gilman, 1980。 Gilman et al., 1981, 1982, 1985。 Arthur et al.,1985). The damage occurs primarily due to oxygen de?ciency in the root zone resulting from a direct displacement of oxygen by land?ll gas. In the absence of a gas control measure, land?ll gas can migrate upward due to concentration and pressure gradients, and escape into the atmosphere by venting through the land?ll cover. During this process, oxygen is displaced and plant roots are exposed to high concentrations of methane and carbon dioxide, the two major constituents of land?ll gas. The lack of oxygen causes the death of plants of asphyxia. . Unpleasants Odors are mainly the result of the presence of small concentrations of odorous constituents (esters, hydrogen sul?de, organosulphurs, alkylbenzenes, limonene and other hydrocarbons) in land?ll gas emitted into the atmosphere (Young and Parker,1983, 1984). The odorous nature of land?ll gas may vary widely from relatively sweet to bitter and acrid depending on the concentration of the odorous constituents within the gas. These concentrations will vary with waste position and age, deposi