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S lEC0R • Conceptual Site Model March 24, 2004 Page 9 51 1 Onsite Groundwater Analytical Data A summary of groundwater laboratory results for 4003 is listed below Number of Detections Minimum Maximum - Constituents above the MDL of the 3 Concentration in pg/L(Well ID) Concentration in pg/L(Well ID) Total Samples TPHg 2 620 (MW 2) 6 800(MW-3) Benzene 2 5(MW 2) 500(MW-3) MtBE 3 3(MW-1 & MW-3) 590(MW-2) x Definitions TPHg =Total petroleum hydrocarbons as gasoline MDL = Method detection limit MtBE= Methyl tertiary butyl ether s pg/L = Micrograms per liter r TPHg was reported below the RBSL in MW-1 and MW-2 Benzene was reported below the primary MCL in MW-1 MtBE was reported below the secondary MCL for groundwater samples collected from MW-1 and MW-3 51 2 Offsite Groundwater Analytical Data Groundwater monitoring wells B1 R-MW 1 R (transmission shop well), MW-8, MW-8B, MW-8C (former ARCO wells), MW-7, and MW-6 (former Texaco wells) were evaluated for the presence of petroleum hydrocarbons TPHg, benzene and MISE were reported below laboratory reporting limits In B1 R- MW1R, MW-8, MW-8B, and MW-8C TPHg was reported at concentrations of 77 µg/L (MW-7) and below laboratory reporting limits in MW-6 MtBE was reported at concentrations of 0 57 µg/L (MW-6) and 0 79 µg/L (MW-8C)VTPHg, benzene, and MtBE was reported below RBSLs and MCLs to the north, northeast, and, east The presence of low levels of petroleum hydrocarbons in off-site down- gradient wells suggests that impacts to groundwater beneath the Site have not migrated across West Charter Way and South Lincoln Street 51 3 Estimated Mass of Petroleum Hydrocarbons Remaining in Groundwater To calculate the mass of TPHg, TPHd, benzene, and MtBE remaining in groundwater, the following assumptions were made • To estimate the mass in groundwater maximum concentrations of TPHd, TPHg,624g/L,and MtBE reported for fourth quarter 2003, was assumed (120 gg/L, 6,800 gg/L, 624g/L, 500 V5- and 590 gg/L, respectively), • Concentrations of constituents in the dissolved phase are assumed to vary linearly with distance between isocontour intervals This assumption was made to facilitate a calculation by removing the logarithmic diffusion rate Because concentrations typically decrease exponentially with respect to distance, a linear relationship is likely conservative, and, 0 A porosity of 35 percent was used which corresponds to "silty-sand" (American Society for Testing Materials — D2488) I IChevron1920331REPORTSICSMICSM 92033 (REV [3) doc, s