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24 <br /> in NP-1 is less than that in MW 2 and MW-10 by factors of 3 7 and 2 6, respectively The pH <br /> in NP-1 is greater than the pH in both wells MW-2 and MW-10 by greater than 1 pH unit <br /> Based on these data, the water sampled in NP-1 is of clearly different quality than that <br /> ' m MW-2 and MW-10 Based on the clearly higher concentrations of chloride, sulfate, and <br /> ' TDS, and the higher pH,it appears that the water sampled in NP-1 may represent the influence <br /> of the intruding salt water front documented in the 1997 San Joaquin County Flood Control & <br /> ' Water Conservation Report The saline front documented in that report is defined by a <br /> concentration of chloride greater than 300,000 ug/L Based on that criteria, NP-1 would be <br /> ' mapped as occurring within the zone of salt water intrusion influence <br /> ' 6.2 NATURE OF THE RELEASE <br /> ' PHR Consultants (1992) reported that during the 1967 station remodel, three <br /> previously installed 4,000-gallon underground storage tanks(USTs),two pump islands, and six <br /> dispensers were replaced with two 10,000-gallon USTs and four dispensers on two pump <br /> islands The station was subsequently demolished between February and April 1988 During <br /> station demolition, two 10,000-gallon underground gasoline tanks were removed from a <br /> common excavation along with a separate 550-gallon waste oil tank <br /> The groundwater elevation-data indicate that groundwater beneath the Site was <br /> ' deepest at the beginning of the monitoring data record in March, 1993 (between August, 1990 <br /> and November, 1992, the monitored wells were dry) Since that tune, groundwater elevation <br /> has s#eadily increased Everett(1980)cited a groundwater depression centered in the Stockton <br /> ' area (Figure 3-1) during the Fall of 1964 Examination of the piezometnc contour plot in the <br /> Everett reference indicates that the piezometnc surface of groundwater in the vicinity of the <br /> ' Site in Fall, 1964 was at an elevation of approximately -60 to -70 feet above mean sea level <br /> (msl) This corresponds to a depth to groundwater of approximately 50 to 60 feet in the Site <br /> ' vicinity Comparison of the Fall, 1997 San Joaquin County Flood Control & Water <br /> Conservation District Groundwater Report, to a 1970 groundwater map, shows that <br />