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• In summary, fewer wells were impacted this quarter, concentrations were stable or slightly lower in <br /> some aquifers, and the plume in the I" Riverbank Aquifer expanded slightly while the plume in the <br /> 2°1 and 3"d Riverbank Aquifers contracted somewhat From this information, it appears that the <br /> plume is stable or attenuating gradually <br /> 4 0 HYDROCARBON MASS REMAINING IN PLACE <br /> Section 2 3 describes the methods used to calculate the mass and volume of diesel fuel that remains <br /> in place at the site Table 4 summarizes these calculations Most or all of the hydrocarbons above <br /> the depth of 35 feet were removed during the excavation work that was done in 1989, and little if <br /> any diesel leached below a depth of 70 feet Therefore, this table is a good approximation of the <br /> total volume of diesel that remains adsorbed to the soil We estimate that approximately 3,350 <br /> gallons of fuel remain in place If a figure of 7 1 pounds/gallon is taken as the weight of diesel fuel, <br /> then there are approximately 23,827 pounds of diesel stored in the subsurface <br /> i <br /> Approximately 500 gallons (3,590 pounds) of diesel are present in the Riverbank Formation below <br /> the depth of 60 feet The majority of this is submerged beneath the water table at this time The <br /> remainder(2,800 gallons and 19,900 pounds) is adsorbed to soil particles in the unsaturated zone of <br /> the Modesto Formation <br /> • <br />