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1 <br /> 1 <br /> ARCADIS GERAGHTY&MILLER <br /> 6.2 NATURE OF THE RELEASE <br /> PER Consultants (1992) reported that during a 1967 station remodel, two previously <br />' installed 1,000-gallon underground storage tanks (USTs) and one 550 gallon waste oil UST in <br /> the southeast portion of the Site, and two pump islands were replaced with two 10,000-gallon <br /> USTs and a 280-gallon waste oil UST located to the south of the station building UST <br /> integrity testing performed in 1986 indicated that the tanks tested tight The station was <br />' permanently closed in April 1991 A September, 1992 report by GeoStrategies indicates that <br /> the above-mentioned tanks, along with associated piping, were removed from the vacant lot in <br />' January, 1992 <br />' The groundwater elevation data indicate that groundwater beneath the Site was <br /> deepest at the beginning of the monitoring data record in August, 1994 Slice that tune, <br /> groundwater elevation has steadily increased Everett (1980) cited a groundwater depression <br /> centered in the Stockton area (Figure 3-1) during the Fall of 1964 Examination of the <br />' piezometric contour plot in the Everett reference indicates that the piezometnc surface of <br /> groundwater in the vicinity of the Site in Fall, 1964 was at an elevation of approximately-60 to <br /> -70 feet above MSL This corresponds to a depth to groundwater of approximately 50 to 60 <br />' feet in the Site vicinity Comparison of the Fall, 1997 San Joaquin County Flood Control & <br /> Water Conservation District Groundwater Report, to a 1970 groundwater map, shows that <br />' groundwater levels at the center of the depression rose approximately 30 feet during that 27 <br /> year interval and that the large depression centered on Stockton no longer exists Thus using <br />' groundwater condition brackets the time period in which the service station at the Site was <br /> remodeled and underground storage tanks were removed <br /> Figures 5-2 through 54 indicate that the maximum concentrations in both soil and <br />' groundwater samples collected while drilling and sampling both NP-1 and NP-2 at the Site <br /> occurred at a depth of approximately 60 feet bgs Detectable concentrations of TPHg and <br />' 33 <br />