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r Page 9 July 12, 1991 Job No.400511-E1 r . • Compile and analyze data and prepare an interpretive report. The results obtained from this study(hydrocarbon contaminant levels,stratigraphy,etc.)will act as a reference point to which later analytical results may be compared to assess the effectiveness of the remediation program. • After start up, a DERS technician will perform a source test using a portable flame ionization detector(FID)on each of the new wells to determine the extractable soil vent gas hydrocarbon concentration. The technician will optimize the mass extraction rate of the VES by adjusting the vapor wells'valving. Vapor wells which indicate relatively high levels of hydrocarbons will be used as extraction points. Vapor wel!s showing FID readings less than 100 ppm will be used as air induction wells. 4.1 Proposed Subsurface:Soil investigation and Assemment The vertical and lateral extent of vadose zone petroleum hydrocarbon contamination in the vicinity of the former underground fuel tanks and piping system has been established based on the percent of Lower Explosive Limit (LEL) results reported by 1.T. Corporation,April 18, 1985 and July 12, 1985. The depths and locations of the six(6) proposed vapor extraction wells are based on this and current soil vent gas concentrations from active extraction wells. Soil samples will be collected at 5-foot depth intervals from a split-barrel sampler used for lithologic logging during the drilling of the monitoring wells. Soil samples from the borings wii x:selected for laboratory analysis based on field organic vapor readings. One sample in every 10-foot interval will be submitted for analyses. Samples selected for laboratory testing will be submitted to a California- certified laboratory and analyzed for Total Petroleum Hydrocarbons as gasoline(TPHg by the DHS Method) benzene, ethylbenzene, toluene,and total xylenes (BETX by EPA Method 8020). The soil analytical results from each boring will be used as a reference point to which future soil analytical results may be compared as a method of gauging the progress of the remediation systema 42 Vapor Extraction Assessment DERS proposes installing nineteen(19)2-inch diameter combination vapor extraction/air injection wells at the locations shown on Figure 3. Proposed well details are shown on Figure 5. The well location design was based on chemical analytical data from existing wells,and the estimated radius of influence for each well. The zone of influence produced by the vapor wells will fully encompass the hydrocarbon