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Mr. Harlin Knoll .. <br /> San Joaquin County Environmental Health Division ® Pt ON <br /> 1 May 2001 <br /> Page 2 <br /> Summary of Previous Investigations <br /> In May 1997, Foothill Engineering drilled'.four slant borings in the,vicinity of the:three <br /> remaining USTs.- Tofal,petroleum h'ydrocarhons as gasoline (TPHg) were detected in soil <br /> samples beneath 'Tank #3 at -concentrations up to',16;200 milligrams per kilogram <br /> (mg/kg). No groundwater samples were collected during this investigation. <br /> In December 1998, QST Environmental (QST) drilled soil borings BFS-T and BFS-2 in the <br /> vicinity of the three closed-in-place USTs. Figure 2 shows the boring locations. <br /> Groundwater was encountered in these borings at approximately 28 feet below grade. A <br /> groundwater sample collected from boring BFS-1 contained TPHg at 4,200 micrograms <br /> per liter (µg/I) 'and benzene,�toluene; ethylbenzene, and xylenes (BTEX) at 220 µg/1; 400 <br /> µg/1;' 340 µg/I, and 1,500 µg/l, respectively: Methyl tert-butyl ether (MTBE) was also <br /> detected in groundwater from boring 13'FS-1at.a concentration of 0.78. µg/l. Of the <br /> gasoline-related compounds listed. above, only, xylenes-were detected in a groundwater <br /> sample collected from boring BFS-2 (0:55 <br /> In January 2000, Orion installed :three groundwater monitoring wells (MW-1, MW-2, <br /> and MW73) in the vicinity of the former 'USTs. THe' well locations are shown on <br /> Figure 2. One soil sample was selected and analyzed from each boring during drilling <br /> activities. The soil sample from MW-1, at-a depth of 20 feet below.grade, contained the <br /> only detectable concentration of MTBE at 99 micrograms per kilogram (µg/kg). The soil <br /> samples, analyzed from MW-2and MW'3 for MTBE ;Were below detection limits at <br /> 1I depths of'35'a nd 15 feet-below grade, respectively. TPHg and BTEX were not detected <br /> in any of the other soil sarnpies analyzed from the .borings. Analytical results from the <br /> soil samples are summarized in Table 3. <br /> In January and, May 2000, groundwater samples.from well MW-1 (installed near the <br /> former source area) contained,no detectable amounts of TPHg or BTEX. A small amount <br /> of MTBE was detected in MW-1 at a concentration of 7.9 µg/l during the first round of <br /> sampling and 4.0 µg/I during the .second round. Upgradient well MW-2 contained <br /> TPHg concentrations of 91` µg/[during the first round'of sampling and 440 µg/l from the <br /> second round; `MTBE was not detected in either round of sampling. . The samples from <br /> cross-gradient well MW-3 did not contain detectable concentrations of TPHg, BTEX, and <br /> MTBE during both rounds of groundwater sampling. <br /> In August 2000 and January 2001 'groundwater samples from well MW-1 contained no <br /> detectable amounts of TPHg or BTEX. A small'.amount of MTBE was detected in MW-1 <br /> at .a concentration of 7.7'and 42- µg/I,. respectively: Upgradient, well MW-2 contained <br /> TPHg concentrations of.440 and 170 µg/!,.respectively; MTBE was detected in August <br /> 2000 at a concentration of 8.6 µg/I and not'detected in January 2001 sampling. <br /> fr. <br /> SABMSTOCKTONUMI-TRO08.DOC 511/01 <br /> ire <br />