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Ims. 'M <br /> Subsurface Investigation December 12, 1990 <br /> Former Unocal Station No. 2859, Stockton, California AGS 38012-5 <br /> Applied GeoSystems has evaluated the direction of groundwater flow to be northeast since June <br /> 1988. Our methods for evaluating the direction of groundwater flow,evaluating the groundwater <br /> gradient,and constructing the Potentiometric Surface Maps are presented under Field Procedures. <br /> The Potentiometric Surface Map dated May 30, 1990(Plate P-15) is our most recent evaluation <br /> of the ground water gradient and flow direction. In August 1990,we could not measure depths <br /> to water in wells MW-1 through MW-3 because the wells were dry; therefore the gradient was not <br /> evaluated. <br /> The average gradient since June 1988 has been 0.002. Table 6 summarizes ground water <br /> elevations and gradients. <br /> Field data from quarterly monitoring events indicate the monitoring wells will sustain pump rates <br /> ranging from 0.15 to 0.80 gallons per minute. Total volumes of water purged range 14 to 38 <br /> gallons after pumping at least three well volumes. Draw down in the wells ranged from 0.5 to 2.6 <br /> feet and the recovery times were 10 minutes or less. These are minimum values, and do not <br /> represent the maximum yield of the aquifer. <br /> The Map of Total Petroleum Hydrocarbons as Gasoline (Plate P-16) shows the lateral extent of <br /> TPHg plume using the data from the May 30, 1990 monitoring event. The southern and western <br /> _ limits of the dissolved hydrocarbons are not defined with the present data. The Map of Benzene <br /> Concentvations(Plate P-17) also shows approximately the same lateral limits as the TPHg map. <br /> 18 <br /> u . <br /> AVA62a( GOOSYS ` Y77S <br />