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Extended Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 44 For most components of fuel hydrocarbons it would be necessary to consider the applicable Retardation Factor when estimating the hydraulic conductivity from field measurements of groundwater gradients and analyte migration rates. The Retardation Factor for a particular chemical can be defined as follows: R= Vwater/Vcoc (Equation 4) where: R=Retardation Factor Vwater=Rate of Flow of Groundwater Through the Subsurface and Vcoc=Rate of Migration of a Chemical of Concern Through the Subsurface The Retardation Factor is a measure of the reduction in the speed of migration relative to that of groundwater that occurs due to adsorptive partitioning of a chemical of concern (COC) between the free flowing groundwater and the surface of soil solids (particularly organic matter), and reductions in migration rate due to other physicochemical phenomenon including metabolization. Bioremediation of a COC can also destroy a COC by converting it into innocuous materials before it arrives at a point in the subsurface ' down gradient from the point where it was released into groundwater. In the case of _;, MTBE, which was used here as an indicator of the maximum migration rate for components of the fuel hydrocarbons released on the 7500 West Eleventh Street property, its Retardation Factor is essentially 1.00. This is because, in so far as is presently known, i it does not partition between soil and groundwater, it metabolizes very slowly and it is not bioreinediated by biota commonly present in the subsurface beneath release sites. I' = Although the mobility of MTBE in groundwater can explain the fringe of that chemical that precedes the main body of the primary plume of affected groundwater shown on Figure 13, some questions remain regarding the down gradient extent of that plume when viewed in the light of groundwater-quality data obtained from monitoring wells installed on the Suburban Propane property at 23901 Chrisman Road in 1996. (See Figure 13 for locations.) When the monitoring wells formerly located on the Suburban Propane site were sampled on February 19, 1996 and again on April 5, 1996, groundwater in them was found to contain no detectable concentrations of TPH(g) nor any of the BTEX ._-.1 compounds. No analyses were made for diesel-range petroleum hydrocarbons or for MTBE. Taking K= 1.5 x 10.2 cm/sec as estimated above, with the observed groundwater gradient of 0.003 ft./ft., and making the same assumptions as used previously regarding the porosity of the soil and the Retardation Factor for MTBE, in the approximate period of 3.3 years that passed between the assumed October 1992 release of gasoline containing ` that chemical from piping on the 7500 West EIeventh Street site and the February 1996 _--i SJC -�i