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1 February 14, 1997 Page 5 , . 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 included in Attachment A Between July 1991 and June 1993 Groundwater Technology, Inc (GTI) completed five groundwater monitoring wells (MW-9 through MW-13) and one SVE well (V-6) The groundwater monitoring wells were completed to a maximurn depth of 75 feet bgs and SVE Well V-6 was completed to 29 feet bgs TPPH-g and benzene were only detected iii 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 (October 13, 1993) to predict the downgradient concentrations of dissolved benzene from Monitoring Well MW-1 1 (Attachment B) Well MW-1 1 had consistent benzene hits since its devel- opment 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-1 1, 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-1 1, and the degradation rate for benzene were calculated The assumptions for the model were steady-state site conditions, no additional hydrocarbon mass added, and a constant degradation/attenuation rate 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-1 I than were actually observed Chevron suggested that this may be due to another source area, possibly on or off site I The outcomes predicted by the model were as follows 1 The estimated distance downgradient from Well MW-11 to the 10 parts per billion (ppb) concentration contour would be 165 feet 2 The estimated time it would take to degrade the initial dissolved benzene concentration in Well MW-11 (1,500 ppb) to 10 ppb would take 7,197 days or 20 4 years December 1993. Three 50-foot confirmatory soil borings (S13-1, SB-2, and SB-3) were drilled by GTI near the former 10,000-gallon gasoline UST complex to determine the effectiveness of the SVE system (Figure 2) In addition, two 66 5-foot borings were drilled and converted to Wells MW-2R and MW-14 to replace Well MW-2, and to 1201337DCLOSITRE3