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the structures Underpinning the buildings would be very costly No excavation will be conducted beneath the buildings Prior to excavating in the vicinity of monitoring wells (the excavation could potentially impact wells MW-4, MW-5, MW-1A, MW-2A, and MW-3A), the wells will be protected by surrounding them with a steel conductor casing2 to prevent damage to the PVC casing If affected soil extends below the water table, an attempt will be made to remove these affected soils Because the ground water horizon is located within the clay/silt layer, it is anticipated that the first several feet will be dry enough to excavate Near the clay/silt layer and sand layer interface, the soil will most likely be much to wet to excavate The extent of the affected soil has been delineated with soil borings Affected soil containing more than 100 mg/kg TPH as gasoline, or 10 mg/kg total BTEX compounds will be removed. Based on the soil boring data, the excavation area will be approximately 20 feet by 20 feet for Site #1 and 40 by 70 feet for Site #2 The proposed excavation areas are shown in Figure 5-1 Clean sods, including the fill above the visqueen liner at Site #2 will be segregated from the affected soils and stockpiled separately The clean stockpiles will be sampled to confirm that TPH and BTEX concentrations are below action levels, and will be used as backfill in the excavated areas During the excavation, visual and olfactory evidence of the presence of gasoline will initially be used to establish the required extent of the excavation The excavation will also be monitored with a direct- reading instrument (FID) An on-site, California-certified, mobile laboratory will then be used to analyze soil samples to confirm that sufficient soil has been removed These samples will also serve as confirmation samples for the closure The confirmation sampling ' protocol is described in more detail below ' Excavation Confirmation Sampling As indicated above, a mobile, California-certified laboratory will be brought on site to analyze soil samples during the excavation period Use of a mobile laboratory will result in improved delineation of the required extent of excavation, because immediate data will be available Samples collected from the excavation will be analyzed for TPH (gasoline and/or diesel, as appropriate) by EPA Method 8015-Modified, and for BTEX by EPA Method 8020 Because none of the samples 2 Another method may be used if appropnate I 5-3