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Report: Groundwater-qualityMonila°ing—January20,2003: 7500 West Eleventh Street, Tracy, C,4. Page 13 It is also instructive to examine the chromatograms from the analyses of Sample Nos. MW-12 and MW-12AS, copies of which are included in Appendix C. In the chromatogram for sample MW-12A, several small spikes in the diesel range (i.e., the range that covers C9 to C24 carbon-chain molecules) can be seen between the 15.00 min. and 17.53 min. time markers. There are no other significant peaks within the diesel range, but there are several that appear in the adjacent C24 to C36 range. (Note: The spike that rises to the full height of the chromatogram close to the 14.7 min. time marker is the signature of o-terphenyl, the surrogate introduced into the sample by the laboratory for analytical quality control purposes.) r Because they fall with in the diesel range the laboratory, in reporting the results of the analysis of sample MW-12A, quantified the compounds for which spikes appear between the 15.00 min. and 17.53 min time markers as having a total concentration of 100 4g/L, but as is noted on the Certificate of Analysis for that sample, the signature seen on the chromatogram did not match the laboratory's standard for diesel. '= = By comparison, the chromatogram for Sample No. MW-12AS (which was prepared with silica gel, but was otherwise identical to Sample No. MW-12A) has no spikes in the C9 to C24 range, except that associated with o-terphenyl, and only a few very small spikes jremain in the C24 to C36 range. When taken together with the results of the analyses for other fuel hydrocarbons that were performed on Sample No. MW-12A, these data show that groundwater recovered from Monitoring Wells MW-3A, MW-3B and MW-12A, which are screened in the deeper aquifers beneath the site, is not affected by fuel hydrocarbons. Analytical results that indicate the presence of organic compounds having signatures { within the same range as fuel and other petroleum hydrocarbons are not uncommon. In fact, the author of this report has served as the Engineer of Record for three sites in the 4 Tracy area that are not very far distant from the 7500 West Eleventh Street property where significant concentrations of compounds having carbon-chain molecules within the diesel range have been detected in groundwater, but which, on further examination, were found to be attributable to natural, non-petroleum, organic compounds present in the subsurface. r..1 Based upon the evidence presented above, SJC concludes that the deeper confined or - 1 semi-confined aquifers beneath the 7500 West Eleventh Street site are presently free of any petroleum hydrocarbons. However, as is discussed in Section 4.0, we remain f<: concerned that Monitoring Wells MW-3A, MW-3B and MW-12A, all of which penetrate through a clay aquitard that protects those deeper aquifers, or in the case of Monitoring Well MW-3B that penetrates two protective aquitards, represent a threat to the j groundwater quality in the deeper aquifers. Those wells are located in a yard that is daily j trafficked by numerous large tractor-trailer vehicles that could leak fluids, or, as has j happened on this site in the recent past, could spill sufficient gasoline or diesel from a breach in a fuel tank to induce an enviromnerztal emergency requiring an immediate field response. E Sic