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TCA and methylene chloride were detected in elevated concentrations in the landfill gas characterization test and in the integrated surface sample as well. This suggests that the potential for documenting an air release of TCA and methylene chloride from the site is high. For gas migration studies, six probes were installed within 50 feet of the active fill area. The Calderon Guidelines require that one gas migration probe be installed for each 1000 feet of site perimeter. CHSL has a perimeter of about 5,500 feet so six probes were placed at the edges of the site. For a detailed map of the locations of the landfill gas probes, see Figure 3-4 (2). Methane concentrations in probes P-2 through P-5 in the southern perimeter of the landfill ranged from .0002 percent by volume to .4 by volume (2 to 4000 ppm). Probes P-1 and P-6, near the entrance of CHSL, showed higher levels of methane, 50 and 30 percent by volume, respectively. Such high levels suggest that there is a possibility for off-site migration of methane (2). Gas probe results are listed in Appendix D. The City of Tracy is located over 4 miles north of the landfill. The land within a 4-mile radius of the site is sparsely populated; the main population center is the town of Carbona, California, located approximately 3 miles northeast of the landfill. Carbona has a population of 150. The overall air route target population within a 4-mile radius of the site probably numbers less than 1000 (8). 3.6 PROPOSED REVISED HRS CONSIDERATIONS There appears to be a low potential for on-site exposure to refuse at CHSL. The site is fenced and waste haulers are load-checked before being permitted to enter. The facility's eastern side appears to be less secure, being fenced off by a low, barbed wire enclosure. The entire site is adequately posted (7). 3-14 sa/tm/chl/si