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Chevron Products Company ZS <br />Former Chevron Station 9-5775, 301 W Kettleman Lane, Lodi, CA July 24,2000 <br />• • Concentrations of petroleum hydrocarbons and fuel oxygenates in monitoring wells are <br />representative of the upper 45 feet of the dissolved plume This assumption is based on <br />the deepest recorded detection of MtBE in groundwater (MW -10B, screened from 85' to <br />90 feet bgs) Groundwater is typically encountered at approximately 55 feet bgs The <br />assumption that the thickness of the dissolved plume is uniformly 45 feet is considered <br />extremely conservative <br />0 <br />• Concentrations of constituents in the dissolved phase are assumed to vary linearly with <br />respect to distance between isocontour intervals This assumption was made to facilitate <br />calculation by removing the logarithmic diffusion rate Because concentrations <br />typically decrease exponentially with respect to distance, a linear relationship is likely <br />conservative <br />The estimated overall porosity of site soils is 30% Porosity of the various soil types <br />beneath the site range from 30% for sands to 29 9 to 40 7% for clays (Table 5) An <br />overall porosity of 35% has been selected to conservatively estimate the amount of <br />water available in saturated soils Geotechnical analysis results are available in <br />Attachment M <br />The highest representative dissolved hydrocarbon concentrations in the source area are <br />51,000 ug/L for TPHg, 39,000 ug/L for benzene, and 720,000 ug/L for MtBE These <br />values are based on monitoring well data <br />Based upon these assumptions and known densities of gasoline, benzene, and MtBE, the total <br />mass of each constituent has been calculated in the dissolved plume as follows <br />Constituent <br />Hydrocarbon Mass (lbs) <br />Gasoline Hydrocarbons <br />55 <br />Benzene <br />42 <br />MtBE -T <br />137 <br />Soil Mass Balance Calculations <br />Estimates of the total mass of TPHg, benzene, and MtBE have been calculated based on the <br />developed area isocontours (Figures 13 through 15) Calculations are presented in Attachment <br />L Assumptions used for this mass estimate are listed below <br />The area of impacted soil at the site consists of one contiguous area, as shown in Figure <br />7, <br />• Impacted soil in the area of the former UST extends to a total depth of 55 bgs, <br />• Soil concentrations in the former UST area are equal to soil data from borings (EX -1, <br />SB -2, SB -3, and VE -1 through VE -4) <br />077 41991 04319-5775PAR doc <br />