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Mr Harlin Knoll C) RION San JoaquinY Count Environmental Health Division 7 July 2000 Page 2 ! Summary of Previous Investigations In May 1997, Foothill Engineering drilled four slant borings in the vicinity of the three ' remaining USTs Total petroleum hydrocarbons as gasoline (TPHg) were detected in soil samples beneath Tank #3 at concentrations up to 16,200 milligrams per kilogram (mg/kg) No groundwater samples were collected during this investigation In December 1998, QST Environmental (QST) drilled soil borings BFS-1 and BFS-2 in the ' vicinity of the three closed-fn-place USTs Figure 2 shows the boring locations Groundwater was encountered in these borings at approximately 28 feet below grade The groundwater sample from boring BFS-1 contained TPHg at 4,200 micrograms per liter (µg/I) and benzene, toluene, ethylbenzene, and xylenes (BTEX) at 220 µg/I, 400 µg/I, 340 µg/I, and 1 ,500 µg/l, respectively Methyl tert-butyl ether (MTBE) was also detected in groundwater from boring BFS-1 at a concentration of 0 78 µg/1 Of the gasoline- related compounds listed above, only xylenes were detected in a groundwater sample obtained from boring BFS-2 (0 55 µg/I) Well Installation and Development Following permitting and location review by the City of Stockton, Orion drilled, installed, and developed three groundwater monitoring wells on 24 and 25 January 2000 (designated as MW-1, MW-2, and MW-3 on Figure 2) Orion marked the well location on the site and contacted Underground Service Alert to clear the boring locations of subsurface lines and utilities before initiating drilling activities The borings were drilled with 8-inch-diameter, hollow-stem, continuous-flight augers during a heavy rainstorm Soil samples were collected with a California split-spoon sampler that contained three 2- by 6-inch brass tubes The boring logs and well completion diagrams are included in Attachment 1 The well construction diagrams are presented as Figure 3 The borings were drilled to a depth of approximately 36 5 feet below grade Water was encountered at approximately 25 feet below grade New 2-inch-diameter well casings were installed to a depth of 35 feet below grade, with 20 feet of screen from 15 to 35 feet below grade The wells were constructed using new flush-threaded Schedule 40 polyvinyl chloride (PVC) casings with 0 010-inch slotted screen The annular space around the well screens were filled with a filter pack of RMC 2/12 Lonestar filter sand to about 3 feet above the top of the screens An approximately 3-foot-thick layer of bentonite was placed above the filter pack to provide an annular seal After placement, the seal was hydrated with potable water �3 5\BFS\STOCKTON\LTR\LTROOA DOC 7/7/00 J