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URS Mr. Robert McClellon October 22, 2004 Page 5 • A remedial action alternative that could be easily expanded as additional information is collected since the extent of the methane plume along the northern site boundary is not well established. • The cost effectiveness of the remedial action alternative. Based on the above criteria,a passive venting system using vertical ventilation wells was selected as the recommended remedial action alternative. REMEDIATION PLAN This section describes the steps to be completed as part of the site remediation plan. The first step is to establish the methane plume's western extent,along the northern site boundary. The second step will be to identify, screen, and further evaluate the remaining remedial action alternatives (Table 1).The following step would be to implement the remedial action alternative selected during step two(as described below). Additional Site Investigation Activities The results of the October monitoring event,confirmed that the methane gas concentration along the northern property boundary exceeds the LEL. However,the western extent of the methane plume along the northern site boundary has not been established. Additional probes shall be installed west of probe VW-5 to provide additional information for the plume's western extent. URS proposes to install two additional sets of dual completion probes, one at the site's the northwestern corner and one due south of it along the western site boundary.We propose to install these additional probes as per the methodology described in the work plan dated September 13,2004 and the work plan addendum dated October 4, 2004. The proposed locations of the probes are shown on Figure 1. Remedial Alternative Implementation Activities In addition,URS proposes to install two vertical ventilation wells to provide the LFG being generated by the landfill a preferred path to relieve any gas pressures building up below the ground surface. The proposed locations of the ventilation wells are shown on Figure 1. One will be located halfway between VW—4 and VW-5. The second ventilation vent will be located halfway between VW-5 and VW-6. The procedures to install these ventilation wells are specified in the work plan dated November 17, 2003 and the work plan addendum dated December 12, 2003. However,the ventilation well casings will be constructed with 6-inch PVC, in place of 1-inch PVC.The depth of each ventilation well may vary based on the soil lithology found within each boring. Each ventilation well will be screened within the most permeable zone. The locations of the ventilation wells may be changed as additional site information is collected.The SJCEHD will be contacted if the locations of these ventilation wells are changed more than 50 feet,for its prior approval. Initially,the ventilation wells will be operated passively. If it is determined after three weekly monitoring events that their passive operation are not affecting a reduction of the methane gas concentrations in the perimeter probes,URS will conduct a short-term vapor extractionilop t test,for - up to one week. This test will include using the two ventilation wells while monitoring the methane K:\Wprocess\25532\WorldEnterprise\Remediation Plan\Lettendoc