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Mr Kyle Christie Project 20805-144 004 July 26, 1996 Page 5 encountered petroleum hydrocarbon impacts within the backfill, however, impacts were not encountered in soil samples collected at depths of 21 to 31 feet bgs This suggests the impacts are vertically restricted Impacts in the vicinity of the piping trenches and dispensers encountered at depths of 2 to 4 feet bgs, the presence of the lower permeability silt unit would inhibit vertical petroleum hydrocarbon migration The absence of petroleum hydrocarbons in soil samples collected from borings adjacent to the dispenser and piping locations provides evidence of the limited vertical extent of impacts The extent of impacts and mass calculations are based on 1992 analytical results and do not consider the mass removal by biodegradation processes within the soil column Decay rate constants cited in the literature (1995, Buscheck and Alcantar, Proceedings of the 1995 Battelle International Symposium on In Situ and On-Site Bioreclamation) range from 0 001 to I percent per day removal A typical decay rate constant on the order of 0 05% per day over the four year period since site characterization activities would result in an approximate removal of 52% of the petroleum hydrocarbon mass The calculated mass of 735 pounds may have degraded to approximately 353 pounds (119 gallons of gasoline degraded to 57 gallons of gasoline) In addition, the residual petroleum hydrocarbons do not appear to represent a threat to groundwater With the exception of 0 7 riucrograms per liter of ethylbenzene detected in MW-3 during the October, 1993 monitoring event, petroleum hydrocarbons have not been detected in groundwater The depth to groundwater recorded over the duration of the quarterly monitoring program ranged from approximately 50 to approximately 54 feet below ground surface The vertical extent of soil impact extends depths of 20 feet or less below ground surface and there is an interval of approximately 30 feet of soil separating the impacted soil from groundwater This interval includes low permeability silts that would inhibit migration of petroleum hydrocarbons to groundwater The migration of petroleum hydrocarbons in soil can be grossly estimated (Dragun, J , 1988, The Soil Chemistry of Hazardous Materials) using the following equation �l OOQ 3hcRC �¢ where /3 = depth (yd) Vh, = volume of petroleilrm'i ydrocarbons in barrels (44 gallons) A =area of impact = 900 fe of the former UST complex foot print R = soil retention capacity = 25 for fine sand C = correction factor= 0 5 for gasoline �tV SAC1K\08051440 DOC-461dms 1