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i Mr. Ron Rowe Page 4 Vapor Sample Analytical Results Laboratory reports are included in Attachment D and are summarized in Table 4 Based on the TPPH concentrations, the destruction efficiency of the thermal oxidizer during the October and November sampling rounds was 95% or better. In December the destruction efficiency of the thermal oxidizer was 85%, however, the maximum emission of 2 pounds per day stipulated by ' the San Joaquin Valley Air Pollution District Permit to Operate was not exceeded ' Overall, analytical results of fourth quarter influent samples indicated vapor concentrations have declined from the third quarter 1998. Influent vapor concentrations measured in November and December were significantly lower than in October. ' System Evaluation The VES has been removing hydrocarbons from the subsurface since November 19, 1997 at ' combined flow rates ranging from 270 to 390 cubic feet per minute, including dilution air As of December 31, 1998 the system had been in operation for 5,742 hours Based on graphical ' interpolation between measured influent vapor concentrations, as of December 31 an estimated 10,850 pounds of hydrocarbon contamination had been removed from the soil Table 5 summarizes flow and vapor concentration averages as well as the total hydrocarbon mass removed from the soil Evaluation of Air Sparging to Enhance Vapor Extraction As per the San Joaquin County PHS-EHD directive issued October 21, 1998 Ground Zero reviewed the subsurface lithology at the site in order to evaluate the potential effectiveness of supplementing the existing VES with an air sparging system to enhance hydrocarbon removal ' An extensive review of the numerous boring logs that exist for the site indicates a more or less continuous low permeability zone (ML/MH and CL) separates the deeper high permeability ' saturated sediments where air would be introduced through sparging wells, and the shallower unsaturated units subject to vapor recovery. This relatively impermeable unit would be expected to greatly impede the movement of introduced air to the vapor extraction wells for recovery ' Results could include trapping of hydrocarbon vapors beneath the low permeability unit, lateral movement of trapped vapors and channelling to the unsaturated zone through fissures, macropores or discontinuities in the confining unit 1 RECOMMENDATIONS ' Ground Zero recommends against any additional evaluation of air sparging at this time since the site lithology does not appear to be favorable for effective vapor control and recovery t roelclgmrMgm1998 i 1