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Report: Groundwater-quality Monitoring—April 15, 2003: 7500 West Eleventh Street, Tracy, CA. Page 12 f detected were 170 µg/L of gasoline and 1.6 gglL of benzene, Those results are consistent with the trend of falling concentrations of analytes of concern that have been reported for samples recovered from that well since it was first sampled in April 2002. 2.4.2 Groundwater Quality in the Depper Confined AQuifers No detectable concentrations of any analytes of concern that are components of fuel hydrocarbons were detected in samples of groundwater recovered on April 15, 2003 from t: either of the monitoring wells (MW-3A and MW-12A) screened in the aquifer that lies r- # beneath the first clay aquitard present beneath the 7500 West Eleventh Street property at a depth of approximately 25 ft. (See Figures 3 through 5 for hydrostratigraphy.) This was also the case for the sample recovered from Well MW-3B, which is screened in a second, deeper aquifer, the top of which is at a depth of approximately 35 ft. BGS. As can be seen by examination of Table 2, no components of fuel hydrocarbons have been detected in any of the groundwater samples recovered from the wells screened in the deeper aquifers beneath the site since they were installed in March 2002. An apparent trace of MTBE, at 0.60 gg/L, was detected the sample recovered from Monitoring Well MW-3A on May 11, 2002. However, that result proved to be spurious, based on analyses of samples recovered from that well on April 11, July 29 and October 25, 2002, January ( r 20, 2003 and April 15, 2003 2.4.3 Absence of Diesel Range Fuel H drocarbons in Groundwater Sam les As has been detailed above, no diesel range petroleum hydrocarbons were detected in any of the groundwater samples recovered from the monitoring wells at the 7500 West Eleventh Street site on April 15, 2003. As was also described above, in the case of that sampling round, all sub-sample aliquots of groundwater that were analyzed for organic compounds in the Diesel range were pre-treated using the silica gel cleanup procedure specified by EPA Method 3630. As is cited in the footnotes to Table 4, none of EPA Method 1015M chromatograms obtained from samples of groundwater recovered from beneath the site since groundwater-quality monitoring was initiated were a good match, or even an approximate match, to the laboratory's standard chromatogram for diesel. The hydrocarbons that were detected in the diesel range have generally been at very low concentrations. The propensity of the EPA 8015M procedure to yield false positive results for total petroleum hydrocarbons in groundwater is well established (Army and D. A. Wright, 1997; Foote et al 1997; Zemo 1997b; Zemo and Synkowice 1995). Such false positives are commonly seen and can be due to the presence of non-petroleum hydrocarbons such as polar i biogenic material (often having a vegetative source - Girard and Edleman, 1994) or non- petroleum biodegradation products. When false positives for petroleum hydrocarbons are suspected in the results of analyses of samples of groundwater by EPA Method 8015M, silica gel cleanup by EPA Method ( ; 3630 is routinely employed to distinguish false positives from any actual concentrations f sic