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Mr Harlin Knoll ° San Joaquin County Environmental Health Division R 1 N 24 October 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-in-place USTs Figure 2 shows the boring locations Groundwater was encountered in these borings at approximately 28 feet below grade A groundwater sample collected from boring BFS-1 contained TPHg at 4,200 micrograms per liter (ggd) and benzene, toluene, ethylbenzene, and xylenes (BTEX) at 220 gg/I, 400 µg/1, 340 gg/I, and 1 ,500 gg/l, respectively Methyl tert-butyl ether (MTBE) was also detected in groundwater from boring BFS-1 at a concentration of 0 78 gg/I Of the gasoline-related compounds listed above, only xylenes were detected in a groundwater sample collected from boring BFS-2 (0 55 µg/1) In January 2000, Orion installed three groundwater monitoring wells (MW-1, MW-2, and MW-3) in the vicinity of the former USTs The well locations are shown on � ! Figure 2 One soil sample was selected and analyzed from each boring during drilling activities The soil sample from MW-1 , at a depth of 20 feet below grade, contained the only detectable concentration of MTBE at 99 micrograms per kilogram (µg/kg) The soil samples analyzed from MW-2 and MW-3 for MTBE were below detection limits at depths of 35 and 15 feet below grade, respectively TPHg and BTEX were not detected in any of the other soil samples analyzed from the borings Analytical results from the soil samples are summarized in Table 1 In January and May 2000, groundwater samples from well MW-1 (installed near the former source area) contained no detectable amounts of TPHg or BTEX A small amount of MTBE was detected in MW-1 at a concentration of 7 9 gg/1 during the first round of sampling and 4 0 µg/I during the second round Upgradient well MW-2 contained TPHg concentrations of 91 gg/I during the first round of sampling and 440 gg/l from the second round, MTBE was not detected in either round of sampling The samples from cross-gradient well MW-3 did not contain detectable concentrations of TPHg, BTEX, and MTBE during both rounds of groundwater sampling fThird Quarter 2000 Groundwater Sampling and Analytical Results The monitoring wells were sampled again August 2000 the groundwater sampling and analytical procedures were conducted in accordance with Orion's work plan Depth to • groundwater was measured before sampling using a Solinst water level meter 5\BFS\STOCKTON\LFR\LTR005 DOC 10/24/00