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sample from GT-9 McCampbell analyzed the sample twice, obtaining concentrations of 140 and 150 ppb If both the McCampbell and Excelchem results are accurate, then GT-9 must he near the zero edge of groundwater contamination, and the diesel may have entered the well during development and purging If one of the results is incorrect, possible explanations for the confhctmg results include procedural differences between the Excelchem and McCampbell laboratories, error or cross- contamination by one or both laboratories, and sampling error by Upgradient Except for the hydropunch sample from GT-4, a!1 of the samples were collected from the Riverbank Aquifer The very high gasoline and diesel concentrations detected in GT-4 indicate that most of the contaminants are concentrated in the Modesto aquifer Benzene, ethylbenzene, and xylene were also - detected at elevated concentrations in this sample 5.3 Vertical Extent of Soil Contamination In Figure 10, laboratory data have been superimposed on Appendix Figure A-1 and contoured to provide a cross-sectional view of the extent of soil contamination This procedure could be done on any of the cross sections in Appendix A, but one is sufficient for the discussion that follows Very low diesel concentrations were detected in TB-1 (i e GT-3) and TB-2 at 20 feet and 15 feet, respectively Similar or higher concentrations were detected at these depths along the north and east sidewalls of the excavation when it was deepened and enlarged by SEMCO in 1990 (Table 6) Based on the SEMCO data and Figure 10, the upper depth limit of contamination is approximately 20 feet on . the south and west sides of the former tank cavity, and between 10 and 15 feet on the east and north sides The maximum depth at which hydrocarbons have been detected is 95 feet in boring GT-3, but this may be due in part to the fact that no samples below 70 feet in any other borings have been analyzed Nevertheless, the highest diesel concentrations were detected at or just below the base of the Modesto channel in most borings Because of the variation in depth of incision of the channel, tlus depth ranges from 55 feet in GT-3 to 50 feet in TB-2 and GTA For example, diesel concentrations drop from nearly 15,000 mg/kg at the channel sand/silt bed contact to 323 mg/kg in the silt bed in GT-4 In GT-3, where the silt bed has been eroded, concentrations drop from 18,000 mg/kg in the silty sand at the top of the Riverbank Formation just below the channel to 5,600 mg/kg just above the hardpan layer to 3 mg/kg within the hardpan Note, however, that GT-3 was dnlled and sampled in 1990, and concentrations may have declined since then Exceptions occur in GT-1 and in GT-8, where the highest concentrations do not correspond to the base of the Modesto channel In GT-1, diesel concentrations decrease steadily from 6,500 mg/kg at the top of the Modesto channel to 98 mg(kg just below its base (Table 4) Gasoline concentrations roughly follow the same trend, although they reach a maximum in the center of the channel at 45 feet These results imply downward migration of the contaminants through the permeable channel sand In GT-8, diesel was detected at a concentration of 1,640 mg/kg at the base of the channel, but an even higher concentration (3,620 mg/kg) was detected in the Riverbank Formation at a depth of 65 feet, 17 feet below the channel No contamination was detected between these samples at 50 or 60 feet, implying 12