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• were found during the geophysical investigations The above mentioned tank and product piping were found and later removed The soil around the tank was excavated to a depth of 5 feet bgs Soil samples taken from beneath the bottom of the UST near both ends both were non-detect for BTEX compounds and TPPH-g, total lead was detected at concentrations of 6 and 5 ppm Six more sod borings (SB-4 through SB-9) were drilled throughout the site in May 1994 in suspected tank and piping areas No benzene or TPPH-g were detected in any of the soil samples taken from these borings Chevron U.S.A. Products Company In October 1993, Chevron created a fate and transport model to predict the downgradient concentrations of dissolved benzene from Monitoring Well MW-11 Benzene was chosen as the constituent of concern due to the fact that it is carcinogenic and is the most water soluble component of BTEX compounds 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 degrada- tion/attenuation rate The model was tested agamst actual groundwater data and found to be 95 percent accurate 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-11 to the 10 ppb concentration contour would be 165 feet 2 The estimated time it would take to degrade the initial dissolved benzene concentration m Well MW-11 (1,500 ppb) to 10 ppb would take 7,197 days or 20 4 years I Due to the assumption of the model of steady-state site conditions, the large drop in ground- water elevation (15 feet) over tune at the site requires that the model be reviewed with some reservations 3201337B/CAP AMID DOC A- 4 November 30, 1995