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i <br /> 3.2.4 Petroleum Hydrocarbon'Mass Remaining in Groundwater <br /> { The remaining mass of MTBE in grou'n'dwater is calculated using the flowing assumptions: <br /> 'n within an area estimated b a circle <br /> Dissolved cancentrations;of MTBE are contained y <br /> with a 60-foot radius.centered on well MW5, <br /> The vertical extent of.impacted groundwater extends from the groundwater surface at <br /> a depth of approximately 45=feet bgs to the groundwater-bearing sandy interval at a <br /> depth of approximately-75 feet bgs, ' <br /> • 100% saturation throughout the 30-foot impacted interval <br /> • The calculated average'co ncentration of MTBE is uniformly distributed throughout the <br /> impacted'volume of grouri_dwater <br /> •" Average porosity of 0.35- ..' <br /> • Unit weight of water of 62.4'poundslcubic foot <br /> • 1 µg11 (part per billion) ='1_�pound MTBE/1 billion pounds of water <br /> The average concentration of MTBE is calculated.-as follows using the groundwater analytical <br /> results from the fourth quarter of 2005, and is the average of the five wells with <br /> concentrations above the reporting limit: <br /> MTBE average'(52.1+15..5+.i 1.0+9.2+2.2)/5 = 18.0 µg11 <br /> E 4� <br /> The mass of MTBE remaining in groundwater is calculated as follows: <br /> I` L <br /> MTBE mass ((nr2)(75-453](0.35)(62.4 {b-wFatef)(18.0 Ib MTBE) = 0.13 Ib MTBE <br /> (109 lb-waw) <br /> 16 I <br />