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Chevron Products Company l0 Former Chevron Service Station #9-5775, 301 West Kettleman Lane, Lodi. California June 5, 2001 3.4 Estimated Mass of Petroleum Hydrocarbons In Groundwater • The total mass of TPHg, benzene, and MtBE remaining in groundwater are based on change in mass estimates using area isoconcentration contours and GWE system mass removal estimates. Concentration data from July 24, 2000 was used to develop area initial isoconcentration contours while GWE system data from lst quarter 2001 was used as an estimate of mass removed to date Area isocontours for TPHg, benzene, and MtBE are presented as Figures 1-4,1-5, and 1-6, located in Attachment I Mass balance calculations for soil and groundwater are presented in Attachment J Assumptions used for this mass estimate are listed below Concentrations of petroleum hydrocarbons and fuel oxygenates in monitoring wells are representative of the upper 52 feet of the dissolved plume. This assumption is based on the deepest recorded detection of MtBE in groundwater (MW -19C, screened from 105 to 110 feet bgs) Groundwater is typically encountered at approximately 55 to 58 feet bgs The assumption that the thickness of the dissolved plume is uniformly 52 feet is considered conservative • Concentrations of constituents in the dissolved phase are assumed to vary linearly with respect to distance between isocontour intervals This assumption was made to facilitate calculation by removing the logarithmic diffusion rate Because concentrations typically decrease exponentially with respect to distance, a linear relationship is likely conservative • The estimated overall porosity of site soils is 30% Porosity of the various soil types beneath the site range from 30% for sands to 29 9 to 40 7% for clays (Table 5) An overall porosity of 35% has been selected to conservatively estimate the amount of water available in saturated soils Geotechnical analysis results are available in Attachment K. • The highest representative dissolved hydrocarbon concentrations in the source area are 51,000 ug/L for TPHg, 39,000 ug/L for benzene, and 720,000 ug/L for MtBE These values are based on monitoring well data Based upon these assumptions and known densities of gasoline, benzene, and MtBE, the total mass of each constituent has been calculated in the dissolved plume as follows CONSTITUENT ESTIMATED x a MASS REMOVED'', �. r ,� it MASS REMAINING! PERCENTAGE" HYDROCARBON TO DATE (lbs) -1" IN GROUNDWATER;,,-, REMOVED ,, MASS (ibs)-JULY QUARTER 2001 (Itis) -1ST QUARTER 24, 2000 4001 a Gasoline 55 5.59 4941 10 Hydrocarbons Benzene 42 0136 4186 0.3 MtBE 137 6611 7089 48 I 1Chevron1957751ParFrp19-5775PARFRP2 doc