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Report: Groundwater-quality Monitoring—October 25, 2002: 7500 West Eleventh Street, Tracy, Cil. Page 12 aquitard that is present beneath the 7500 West Eleventh Street property at a depth of approximately 20 ft. See Figures 3 through 5 for h drostrati a b This was also the ' case when those wells were sampled on July 29, 2002. However, in the samples h } recovered from Monitoring Wells 3A, 3B and 12A on October 25, 2002, the laboratory reported the presence of a compound that was NOT a fuel hydrocarbon, but that included components with molecules having carbon-chain lengths within part of the same range that includes compounds that are components of diesel. The concentrations of those molecules in the samples ranged from 100 to 150 µg/L. Because the molecules detected are part of an unidentified compound that does not match the laboratory standard for any fuel hydrocarbon that was released into the subsurface at the 7500 West Eleventh Street site, the concentration of diesel detected in samples of groundwater recovered from Monitoring Well 3A, 3B and 12A on October 25, 2002 has been entered as ND (not detected) in Table 2, but the presence of the unidentified molecules has been noted in the footnotes to that table. j Examination of Table 1 shows that the pieziometric level in the near-surface aquifer monitored by Monitoring Wells MW-3 and MW-12 is slightly higher than the pieziometric heads in the aquifers monitored by MW-3A, MW-12A, and MW-3B. Thus, =_ there is a positive downward vertical gradient, although small, between the near surface aquifer and the deeper aquifers. However, when the stratigraphy and geochemistry of the site are examined (see Figures 2, 4, 5 and Table 2), it is clear that if the source of the J unidentified compound in Monitoring Wells MW-3A, MW-3B and MW-12A had reached the deeper aquifer by downward migration from the shallow aquifer that is affected by components of hydrocarbon fuel, when first detected on October 25, 2002, they should have contained MTBE or molecules in the range typical of gasoline, which compounds migrate through soil at a much faster rate than the longer-chain molecules _# that were observed in the samples. Therefore, it appears unlikely that the apparent contamination of the deeper aquifers seen in the October 25, 2002 samples could have a source in the affected near-surface aquifer beneath the site. SJC and STL have studied the analytical data obtained from each of the samples recovered on October 25, 2002. Based on the field procedures used and the order in which the samples were recovered, SJC can see no evidence that the compounds detected in Monitoring Wells 3A, 3B and 12A could have been introduced into the samples from contaminated equipment. " SJC then considered the possibility that the unidentified compounds in the samples were components of turbine oil used to lubricate pumps in agricultural or domestic wells 4 located in the neighborhood. Contamination of groundwater by oil drip-fed into turbine pumps in agricultural and domestic wells is a common source of groundwater contamination in the Tracy area. However, local inquiry found that turbine pumps were not currently in use in any of the close-by wells and, in so far as it is known, have not been used in those wells in the recent past. SJC also considered the possibility that the deeper aquifers had become contaminated by some unknown material being spilled on the surface and passing downward into the sic " .I