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Extended Site Characterization Report; 7500 West Eleventh Street, Tracy, CA. Page 45 groundwater sampling round conducted on the Suburban Propane site, MTBE would be f estimated to have reached a point some 255 ft. down gradient from the former location of the dispenser pump islands on the subject property. That distance is significantly less than the approximate 440 feet that separates 7500 West Eleventh Street from the Suburban Propane Site. Thus, it can be concluded with reasonable certainty that, by February 1996, gasoline containing MTBE that had leaked into the subsurface on the 7500 West =+ Eleventh Street property in October 1992 would not have not reached the Suburban Propane site. R= 1.0 + r Kd 10 (Equation 5) <J where: the dry unit weight of the soil Kd = a distribution coefficient for the COC with the soil and ; 0 =the volumetric moisture content of the soil. Kd for an organic compound in a specific soil is the soil-water partition coefficient, Ko, for that compound times the fraction by weight of organic carbon�matter in the soil. The Kq, for benzene is reported to be 97 mL/g (Roy and Griffin 1985). For a saturated sand with a porosity of 0.3 and particle specific gravity of 2.6, tfe volumetric moisture ._.� content, 0, is 0.3 and the dry unit weight is approximately 1.56 glcm3(i.e., 97.4lb/fta). If, for the purpose of estimation, the weight fraction of organic matter in the soil is conservatively assumed to be 1%, then by application of Equation 5, the Reduction Factor for benzene in the subsurface beneath the 7500 West Eleventh Street site is estimated to be 5.5. Given the benzen&migration parameters stated above, a release of gasoline could have occurred in 1994 and the benzene it contained would not have reached the southern boundary of the Suburban Propane site until February of 1996 and, thus, would not be present in groundwater in the wells installed on that property that were sampled at that time. r. In the 6.1 years spanning the period between the closure of the monitoring wells at Suburban Propane and installation of monitoring wells by SJC further to the north along Chrisman Road in March 2002, the benzene would have migrated an additional 86 ft. down gradient. Field confirmation of such estimations is not easily obtained because benzene and all of the other BTEX compounds are readily eliminated by intrinsic bioremediation, with the result that they may disappear from groundwater long before - they could have migrated to a specified down-gradient location. -.. sic