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Ms Diane Hinson <br /> September 14, 1994 <br /> Page 3 <br /> The results of the mass calculations are summarized in the following table <br /> ' ARCO Service Station <br /> PARAMETER No 760 No 548 <br /> Estimated total mass of TPHG calculated, 489 383 <br /> pounds <br /> The calculated hydrocarbon mass beneath each site is only an estimate for comparison purposes <br /> only <br /> ' GROUNDWATER FLOW DIRECTION AND GRADIENT <br /> The groundwater gradient and flow direction of the first encountered groundwater beneath each <br /> site was reviewed to evaluate if one of the sites exhibits a greater potential for plume migration <br /> ' ARCO No 760 <br /> A groundwater monitoring program was initiated at the ARCO Lodi site in the first quarter of <br /> 1994, therefore, only two quarters of data were available for review Between March and June, <br /> 1994 the groundwater flow direction has changed approximately 90 degrees, from south-southeast <br /> to east-northeast The gradient has ranged from 0 0013 to 0 004 The third quarter groundwater <br /> ' sampling event is scheduled for the end of September 1994 <br /> ARCO No 548 <br /> ' A groundwater monitoring program was initiated at the ARCO Stockton site in the fourth quarter <br /> of 1991 The groundwater flow direction has been consistently towards the east-southeast with <br /> ' gradient ranging from 0 002 to 0 005 (a gradient of 0 01 was recorded in 1992) The third <br /> quarter groundwater sampling event is also scheduled for the end of September 1994 <br /> SOIL PERMEABILITY <br /> The permeability of subsurface materials encountered beneath each site has been evaluated The <br /> evaluation concentrated on subsurface materials which contain the majority of residual <br /> hydrocarbon mass which could leach from the soil and may further impact groundwater <br />