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4.0 GAS MIGRATION <br />The Guidelines require that ASWAT studies provide sufficient screening <br />information to assess off-site gas migration. EMCON obtained this <br />information by sampling gas probes near the site's perimeter and ana- <br />lyzing the samples. This section describes the probe locations, probe <br />details, sampling equipment, and sampling and analysis procedures. It <br />then presents the analytical results. <br />4.1 SAMPLING LOCATIONS <br />The Guidelines require that the equivalent of one gas probe for each <br />1,000 feet of site perimeter be installed near the perimeter. Foot- <br />hill Sanitary Landfill has a perimeter of about 13,000 feet; there- <br />fore, 13 probes would normally be installed near the site's perimeter <br />to test for off-site underground gas migration. However, as proposed <br />in the workplan, only four probes were installed since the site is in <br />a remote location and the nearest structure is over 0.5 mile north of <br />the landfill. The actual southern site perimeter is further north <br />than originally shown in the ASWAT proposal. Therefore, Probe P-4, <br />located on the southern site perimeter, had to be relocated northward. <br />Figure 1 shows the probe locations. <br />4.2 PROBE DETAILS <br />The probe details are described in the ASWAT proposal (see Appen- <br />dix A). Construction details are provided in Appendix C. <br />4.3 SAMPLING EQUIPMENT <br />The sampling equipment was the same as that used for the landfill gas <br />well testing (see Appendix A). <br />PJ9 9390405.00D 4-1 <br />Rev. 0 09/07/88 <br />Ewon Associates <br />