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s <br /> the statement that even the most mobile of the hydrocarbon constituents are not present in <br /> concentrations which warrant active remediation. The highest concentration of benzene detected <br /> in any onsite monitoring well was: 15 ppb (AW-9; 417192). This concentration is clearly below <br /> levels which would suggest the presence of separate-phase hydrocarbons in ground water beneath <br /> the site. The solubility of benzene in gasoline at 25 degrees C is approximately 72,000 ppb, <br /> assuming a mole fraction of 4% (Feenstra, et al 1991). In addition, Alton Geoscience does not <br /> believe that contamination is not being detected because of site hydrogeology and screen <br /> intervals. <br /> First Bullet Item, Paragraph 6 <br /> Geomatrix states "contamination is likely slowly desorbing from the upper silt unit into the well, <br /> moving down the well due to likely downward gradients during non pumping conditions, moving <br /> outward from the well into the coarser, deeper sand unit, and migrating away from the site in <br /> this deeper unit. " Geomatrix's statement is based on its theory that the aquifer pumped during <br /> the test is confined (page 6 paragraph 1). Significantly, however, the aquifer responded as an <br /> unconfined aquifer, not a confined aquifer, during the pumping test. Geomatrix's theory is <br /> based on the small storativity value estimated for the aquifer, but small storativity values can <br /> also be seen in unconfined aquifers. Thus, contaminant migration due to a downward hydraulic <br /> gradient is not likely. <br /> If Ms. Bice is referring to a vertical concentration gradient, it is also unlikely that contaminants <br /> will move down the well, since,petroleum hydrocarbons are lighter than water. Petroleum <br /> hydrocarbons are typically restricted to the upper zone of the water table, and even during <br /> pumping conditions, will not be likely to move vertically down the well. <br /> Third Bullet Item <br /> The final paragraph in this section discusses ground water data from Boring B-7 as "the only <br /> sample of ground water collected at the water table that has ever been analyzed. " Alton <br /> Geoscience disagrees with this ,statement. As a matter of standard, approved practice, ground <br /> water samples are collected by lowering a bottom-fill bailer to just below the air-water interface, <br /> then carefully transferring the water to the appropriate glassware. All ground water samples <br /> collected at this site have been collected in this manner and under the supervision of the SJC <br /> PHSIEHD. In the case of Boring B-7, this "ground water sample" as well as the "ground water <br /> samples" collected from Boring B-1, B-5, B-6, and B-8 were collected during a qualitative <br /> ground water survey and do not'i conform to regulatory standards for proper ground water <br /> collection in terms of construction ,(surface seals, filter pack, wellhead protection, development, <br /> etc) per Water Well Standards: State of California Bulletin 74-81. Since Boring B-7 was-not <br /> a properly constructed or developed well, the data is not valid and should not be used for other <br /> than qualitative purposes. <br /> 7 <br />