SITE INFORMATION AND CORRESPONDENCE

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FROM 1997, 14 17:44 #888 P.06/24 February 14. 1997 J Page 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 Judy 1991 and June 1993. Groundwater Technology, Inc. (GTI) completed five groundwater monitoring wells (MUT-9 through MW-1_) and one SVE well (V-6). The groundwater monitoring wells were completed to a maximum depth of 7> feet bas and SVE Well V-6 was completed to 29 feet bgs. TPPH 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 (October 13, 1993)to predict the down,adient concentrations of dissolved benzene from Monitoring Well MW-1 I (Attachment B). Well MW-11 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-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 conditions, no additional hydrocarbon mass added, and a constant degradatiordattenuation 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-11 than were actually observed. Chevron suggested that this may be due to another source area, possibly on or off site. The outcomes predicted by the model were as follows: 1. The estimated distance downgradient from Well MW-1 I 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(SB-1, SB-2, and SB-3)were drilled by GTT 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 3201337BCLOSLnM