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Report: Groundwater-quality Monitoring—April 15, 2003: 7500 West Eleventh Street, Tracy, CA. Page 13 7 of total petroleum hydrocarbons that might be present in a sample. Use of the silica gel cleanup procedure for this purpose was approved in 1995 by the technical committee that 4 was convened by the State of California in 1995 to advise on laboratory methods for the detection anduantification of TPH in soil and q groundwater (Zemo, 1947a). Because Monitoring Wells MW-3, MW-3B and MW-12A are screened in relatively deep ' aquifers beneath the 7500 West Eleventh Street site, which hydrostratigraphic, geochemical and hydraulic head data have shown to be isolated from the near surface aquifer, on January 20, 2003, SJC recovered two samples from Monitoring Well MW- ' 12A. One, designated MW-12A, was analyzed for total petroleum hydrocarbons in the diesel range without any preparation. The second sample, MW-12AS, was analyzed by j the same laboratory procedure, but was pretreated by silica gel cleanup before it was injected into the chromatograph. As was reported in the Groundwater-quality Monitoring Report for January 20, 2003 (The San Joaquin Company Inc. 2003), a number of compounds in the diesel range (Cg to C24) were detected at extremely low concentrations. However, the pattern of the chromatogram bore no resemblance to diesel or any other petroleum hydrocarbon. This finding was confirmed by the chromatogram produced by j the analysis of Sample MW-12AS, which was pretreated by silica gel cleanup. That chromatogram contained no trace of any compound in the C9 to C24 range except for a peak associated with o-terphenyl, the diesel surrogate used by the laboratory to perform 1 the analysis. From the foregoing results, SJC is confident that no dissolved petroleum hydrocarbons are present in the deeper aquifers beneath the 7500 West Eleventh Street site. As was noted previously, to evaluate further the potential for false positive results from ., analyses for TPH(d), each of the samples recovered from the monitoring wells at the =' 7500 West Eleventh Street site on April 15 2003 was treated with silica gel cleanup before it was analyzed. As was expected, no TPH(d) was detected in any of the samples. This again demonstrates that the very low concentrations of organic compounds that have previously been detected in samples of groundwater that were analyzed for diesel are not components of that petroleum hydrocarbon fuel. >� Based upon the evidence presented above, SJC concludes that the deeper confined or semi-confined aquifers beneath the 7500 West Eleventh Street site are free of any petroleum hydrocarbons and we remain greatly concerned that Monitoring Wells MW- �. 3A, MW-3B and MW-12A, all of which penetrate through one or more clay aquitards that protect deeper aquifers, represent a threat to the groundwater quality beneath the 7500 West Eleventh Street site. As has been noted in earlier reports, those wells are located in a yard that is heavily trafficked by tractor-trailer trucks that could easily leak fluids. In fact, a complete breach of a truck's fuel tank, that required an emergency response by the SJCEHD, actually occurred on the property about one year after monitoring wells were first installed. Fortunately, that tank breach was located at a sufficient, but not great, distance from a monitoring well .and, due to a timely containment and remediation, no groundwater contamination occurred. I sic ` 1