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Report: Groundwater-quality Monitoring—April 15, 2003: 7500 West Eleventh Street, Tracy, CA. Page 14 However, the SJCEHD has requested that the material seen in the chromatograms for the ,:._::;3 TPH(d) analyses be identified before the monitoring wells that penetrate the deeper aquifers beneath the subject property are closed (San Joaquin County Environmental Health Department 2003). Unfortunately, material identification by gas chromatography relies on comparison of a chromatogram of a detected material with the chromatogram of a known standard. The organic materials seen in the diesel-range chromatograms from the samples of groundwater recovered from beneath the 7500 West Eleventh Street site that were not pre-treated with silica gel are not dissolved petroleum hydrocarbons, but the patterns seen in the chromatograms reflect materials containing a multiplicity of compounds and those -`a materials are not identifiable b as chromatographybecause the match no•known Y g Y standard material. Theoretically, mass spectrometry techniques might be considered to identify some of the multitude of compounds represented in the chromatograms if the spectra of the individual compounds were known. The origin of such materials is ` unknown and they are likely present in such minute quantities that application of mass spectrometry analytic techniques would be impractical. (Their total concentration is -_ typically 130 µglL or less; therefore, the amount of any one compound present in the material would be no more than one order of magnitude less than even that minute r quantity.) Thus, while it can be reliably stated that the organic compounds detected in the samples <, of the deeper groundwater recovered from Monitoring Wells MW-3A, MW-3B and MW- ', 12A are not components of hydrocarbon fuels, no, practical, technical means is available to identify them further. r <-1 t-] 4 sic