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u • The objective of the methane monitoring program is to ensure that the concentration of methane gas generated by the decomposition of refuse within the landfill does not exceed the lower explosive limit for methane (5%) at the migration monitoring perimeter, and that the concentration of methane gas does not exceed 25 percent of the lower explosive limit (1.25%) in on-site structures, in accordance with Title 27, §20919, Gas Control. Points of Compliance Perimeter Migration Monitoring This Plan proposes an Interim Monitoring Perimeter near the limit of refuse within the property boundary of the Foothill Sanitary Landfill. The County believes that a monitoring perimeter within the property boundary is superior to a perimeter at the property boundary because of the ability to detect combustible gas migration long before combustible gas actually leaves the property (Figure 1). The interim nature of this perimeter is based on the need to adjust the perimeter to accommodate future development of the landfill. The perimeter monitoring network consists of nine soil gas wells (SG -2 thru SG -10) installed in July 2003, and one well (SG -12) installed in June 2004 (Figure 1). These wells allow monitoring of subsurface gas VOC concentration in compliance with the Waste Discharge Requirements issued by the Regional Water Quality Control Board, and are proposed to be utilized as perimeter migration monitoring wells. Three probes were installed in well SG -12 (Figure 2) and single probes were installed in wells SG -2 through SG -10 (Figures 3). The soil gas wells address the site factors per Title 27, §20923, as follows: A. Local soil and rock conditions do not require special design features in the soil gas wells. The site materials from the surface to 300 feet in the landfill vicinity consist of alluvial deposits of two formations, the Laguna and Mehrten Formations. The Laguna Formation consists of consolidated gravels, sandstones, and siltstones. The Mehrten Formation consists of sandstone and siltstone, with conglomerate layers, claystone and mudstone. B. Hydrogeolgic conditions at the disposal site do not require special design features in the soil gas wells. Groundwater in the vicinity of the landfill property is continually being recharged by percolation of surface waters which originate in the Sierra Nevada and adjacent foothills. Depth to groundwater ranges between 204 feet and 270 feet. Groundwater level ranges between 46 feet to 52 feet above mean sea level (MSL). The groundwater gradient is 0.0015 foot per foot to the southwest. Foothill Landfill Methane Monitoring 1 Public Works/Solid Waste September 15, 2003 County of San Joaquin