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ATTACHMENT 5: ' ASSUMPTIONS AND CALCULATIONS TO ESTIMATE HYDROCARBON MASS ' Geraghty & Miller has estimated the mass of petroleum hydrocarbons remaining in the soil and dissolved in groundwater Numerous assumptions must be made to estimate the total mass in the ' subsurface A summary of the assumptions and sample calculations is described below Average concentrations of TPH as gasoline found in soil borings, monitoring wells, remediation ' wells, and samples collected from the former tank pit are depicted on Figure 5-1 Average concen- trations were calculated over a 10-foot vertical section containing the highest concentrations found in ' that boring or well Tank pit samples collected from near each of the three former tanks were averaged separately ' Geraghty & Miller then distinguished three zones of average concentrations, as shown in Figure 5-1 The concentrations of each zone are approximated at 1,500 mg/kg for the zone including ' the former tank area, 750 mg/kg for the zone extending to include the former pump islands, Monitoring Well MW-1, and Remediation Wells GM-3 and GM-6, and 100 mg/kg for the remaining zone, extending over approximately half the site This approximation of zones of equal concentrations was utilized to facilitate the estimation of total mass of petroleum hydrocarbons to the subsurface ' These zones are not intended to be interpreted as actual concentrations of petroleum hydrocarbons that would be found in the subsurface ' The mass of petroleum hydrocarbons in each zone is calculated by determining the volume within the zone, multiplying by the bulk density of soil (approximately 100 lb/ft3), and then multiplying by ' the average concentration of petroleum hydrocarbons for the zone A sample calculation for the zone of highest concentrations is presented below ' 25 feet x 35 feet x 10 feet(thickness) x 100 lb/ft3 x 1,500 ppm= 1,312 5 lb ' Calculations for the intermediate zone yield approximately 3,056 pounds, the largest zone contains approximately 1,650 pounds using the above-mentioned assumptions This yields a total mass of ' petroleum hydrocarbons in soil of slightly over 6,000 pounds Groundwater contained in the largest zone shown in Figure 5-1 has a total volume of approxi- mately 481,000 gallons, assuming a 10-foot aquifer thickness This yields a mass of groundwater of approximately 4,000,000 pounds Assuming a high groundwater concentration of 10 mg/L, the total GM RICHMOND DATA CHEVRON CHEV N Z"ENJ"CORK-ICT Attch5 d" GERAGHTY& MILLER,INC ��