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y � September 18, 1996 Page 4 r January 1989. EA drilled seven borings and completed them as five soli vapor_extrac- tion (SVE) wells (V-1 through V-5), and two groundwater monitoring wells (MW-7 and MW-8) ' The maximum concentration of petroleum hydrocarbons were reported at 26 5 feet bgs in the sample collected from Boring V-3 (16,000 ppm TPPH-g and 160 ppm benzene), drilled through the center of the former l 0,000-gallon gasoline tank complex (Figure 2) April 1989, EA conducted a pilot-scale test utilizing SVE Wells V-2 through V-5 to determine the design parameters for a vapor extraction system to remediate areas of hydrocarbon-Impacted soil on site Vapor samples from Wells V-2 and V-5 contamed concentrations of TPPH-g up to 4,500 and 16,000 ppm, respectively, and a radius of Influence of at least 22 feet was calculated indicating that SVE was a practical approach x November 1990. SVE was initiated The SVE system was operated from November 1990 until November 1992, with a 7 month hiatus from February to September 1992 Results of SVE operation are presented in a table mcluded,in Attachment A Between July 1991 and June 1993. Groundwater Technology, Inc (GTT) completed five groundwater monitoring wells (MW-9 through MW-13) and one SVE well (V-6) The groundwater monitoring wells were completed to a maximum depth of 75 feet bgs and SVE Well V-6 was completed to 29 feet bgs TPPH'g and benzene were only - detected in the samples collected from the borings for Well MW-9 and'SVE Well V-6 The sample collected at 26 feet bgs in the boring for Well MW-9 located proximal to the northern corner of the former UST complex reported the maximum concentration of petroleum hydrocarbons at 3,900 ppm TPPH-g and 1 7 ppm benzene (Figure 2, Table 2) October 1993. Chevron created a fate and transport model to predict the downgradient concentrations of dissolved benzene from Mormtonng Well MW-11 Well MW-11 had consistent benzene hits since Its development in May 1992 until October 1993 The model used by Chevron estimated the half life of the dissolved benzene and the total migration distance for benzene from Well MW-11, the most downgradient monitoring well on site Using the following assumptions, the velocity of groundwater, velocity of benzene, distance of benzene impact from Well MW-11, and the degradation rate for benzene were calculated The assumptions for the model were steady-state site condi- tions, no additional hydrocarbon mass added, and a constant degradation/attenuation rate r , The model was tested against actual groundwater data and found to be 95 percent accu- rate for Well MW-3, however the model predicted much lower benzene concentrations at Well MW-11`than were actually observed Chevron suggested that this maybe due to another source area, possibly on or offsite The outcomes predicted by the model were as follows 3201337B1CLOSLTRE I ` ,