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Conditions and Assumptions for Contaminant Mass Calculations 1 Reported Total Petroleum Hydrocarbons as gasoline(TPHg) concentrations are assumed to be equivalent to reported Gasoline Range Organics (GRO) concentrations for simplicity in comparing historic versus present values 2 Historic soil and groundwater concentrations reported between 1987 and 1993 are assumed to represent the site conditions upon completion of the 1993 site investigation, and are used as a single representative data set 3 TPHg soil impacts are assumed to only occur below 7 feet bgs,based on depths reported for soil samples in Attachments C through E 4 Site soils are assumed to have an average bulk density of 120 pounds per cubic foot This value is estimated based on standard soil density ranges 5 Site soils are assumed to have an average porosity of 0 35 This value was estimated based on boring log descriptions of soils and standard porosity ranges for the soil types 6 ATR-5 and ATR- 8 concentrations reported in the fourth quarter 2004 are combined with first quarter 2004 data from the other site wells to estimate contaminant mass Groundwater analytical results for all site wells were last reported in the first quarter of 2004 Since that event, only ATR-5 and ATR-8 have been sampled Current mass Iestimates are based on the most recent available data from each well 7 Based on the mass removal estimate, and performance data for the SVE system, and historic soil removal from the site, it is assumed that all remaining TPHg contamination beneath the site is restricted to groundwater 8 MtBE was not analyzed in any samples collected at the site prior to 1996 No historic mass estimates are made for MtBE due to insufficient data 9 Iso-concentration contour figures presented to this attachment are assumed to be representative of site conditions at the time intervals indicated and previously described