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Hydrocarbon Mass Calculations for Tosco (76) Service Station No. 11192 A) Hydrocarbon Mass Calculations in Groundwater Assumptions 1 Mass of impacted groundwater is equal to the mass of soil times the porosity of the soil An area approximately equal to the lowest iso-concentration contour is used, times the thickness of impacted groundwater which is equal to 17 feet (depth to water (— 13') to average depth of wells (-30')) 2 Porosity of soil in saturated zone is approximately 35% 3 Mass = Volume x Density x Concentration TPH� Volume (V) = 45' x 75' x 17' = 57,375 ft3 x 0 35 x 62 4 lbs/ft3 = 125 x 106 lbs water Average TPHg concentration = (140+150)/2 = 145 ppb = 0 145 ppm Mass of TPHg = (1 25 x 106 lbs x 0 145 lbs TPHg)/1x 106 lbs H2O = 0.18 lbs TPHg in water Benzene Volume = 60' x 40' x 17' = 40,800 ft3 x 0 35 x 62 4 Ibs/ft3 = 8 9 x 105 lbs water Average Benzene concentration = (11+12)/2 = 11. 5 ppb = 0 0115 ppm Mass of Benzene = (8 9 x 105 lbs x 0 0115 lbs Benz)/1x 106 lbs H2O = 0.01 lbs Benzene in water MTBE Volume = 250' x 150' x 17' = 637,500 ft3 x 0 35 x 62 41bs/ft3 = 139 x 107 lbs water Average MtBE concentration = (2 7+5 4+55+460+800)/5 = 265 ppb = 0 265 ppm Mass of MtBE = (1 39 x 107 lbs x 0 265 lbs MTBE)/Ix 106 lbs H2O = 3.69 lbs MTBE in water TBA Volume = 170' x 95' x 17' = 274,550 ft3 x 0 35 x 62 4 lbs/ft3 = 5 99 x 106 lbs water Average TBA concentration = (3600+220000+240000)/3 = 154533 ppb = 155 ppm Mass of TBA= (5 99 x 106 lbs x 155 lbs TBA)/1x 106 lbs HZO = 929 lbs TBA in water • 140245 08