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November 12, 1996 Page 5 October 1993. Chevron cleated a fate and transportmodel (October 13; 1493) to- predict the downgradient concentrations of dissolved benzene from Monitoring Well MW-I 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 WeII MW-1 ], the most downgradient monitoring well on site Using the following assumptions, the velocity of groundwater, velocity bf benzene, distance of benzene impact from Well MW-I ], 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 degradationlattenuation 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 oor off site The outcomes predicted by the model were as follows I The estimated distance downgradient from Well MW-I I to the 10 parts per billion (ppb) concentration contour would be 165 felt 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 GTI near the former I0,000-gallon gasoline UST complex to deterrmne the effectiveness of the S VE system (Figure 2) In addition, two 66 5-foot borings were drilled and converted to Wells MW-2R and MW-I4 to replace Well MW-2, and to further define the downgradient extent of petroleum hydrocarbons in groundwater, respectively Of the 30 soil samples collected, the maximum concentration of petroleum hydrocarbons of 2,000 ppm,TPPH-g was reported at 25 feet bgs in Boring SB-2 and also at 30 feet bgs in Boring SB-3 No benzene was detected 1n any samples analyzed (Figure 2, Table 2) While drilling the boring for Well MW-2R, one additional unknown UST was encoun- tered at a depth of approximately 4 feet bgs near the southeast corner'of the site (Figure 2) GTI retained Spectrum Environmental Services, Inc to conduct a geophysi- cal survey of the former service station property in an effort to locate other USTs that were not identified previously No other USTs were found during their investigation March 1994. Touchstone Developments excavated and removed one I,000-gallon UST and associated product piping from the site One soil sample was collected at 8 5 feet 3201337MCLOSUiLE2 yt � I i.