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KLEINFELDER 5.1.3 Subsurface Migration Of Landfill Gas Four landfill perimeter monitoring probes were installed on each edge of the existing landfill. Samples were collected from each probe into tedlar bags. These samples were then screened in the field using an OVA 128 to measure total organic compound concentrations. Emcon Associates .reported total detectable organics measured as methane of between 2 and 3 ppm in two probes 9 percent in the southern perimeter probe and 24 percent in the eastern perimeter probe. This indicates a potential for offsite landfill gas migration, however, no residential housing exists near where these probes were located (east and south sides). 5.2 POTENTIAL TO AFFECTED GROUND WATER QUALITY Concentrations of organic compounds were detected in gas samples collected from the fill. In evaluating the potential for landfill gas migration to the ground water, the following conditions should be considered. Ground water occurs at a depth of a least 200 feet below the landfill. Although little research has been published on the transport of landfill gas, it appears unlikely that landfill gasses would migrate 200 feet vertically downward through the various sediments to the ground water rather than migrating laterally or vertically upward through the landfill cover. _ If gasses did migrate to the ground water, they could dissolve in the ground water and impact ground water quality. This impact may include lowering the pH of the ground water as carbon dioxide in the gas dissolves into the water. Other mechanisms by which landfill gas could potentially impact ground water quality could include: o Condensation of gases (creation of "leachate") and subsequent downward migration of the leachate o Dissolution of gasses into landfill leachates and subsequent downward migration of the leachate. FOOTHILL 33