SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Geomatrix states "If water levels'`continue to rise at the site, these contaminants will be spread into the clean fill above, " Alton Geoscience notes that no real water level rise has been observed at the site since monitoring began 3 years ago. In addition,;the clean fill above is comprised of crushed rock at the soil/water interface (crushed rock was used as the first lift when backfilling), making it highly improbable that a rise in water levels would cause contaminants to be adsorbed by:the fill material. Section 1.2.6.2, Pages 1-7 and:1-8. First Bullet Item, Paragraphs 1, 2, and 3 Monitoring Wells AW-10, AW-11, and AW-12 were constructed with a'screened interval from 5 to 25 fbg. AW-9 was constructed with a screened interval from 8 to 28 fbg which intercepts the shallow, perched aquifer similarly. Differences in "aquifer response" from well to well at this site reflect natural variances in hydrogeological conditions from well to well. It should be noted that wells AW-9 through AW-12 were terminated within a clay, and that in AW-9, the boring log indicates that from 25 to 28 fbg, a stiff, silty clay is present. - It is not expected that 3 feet of stiff, silty clay would contribute an appreciable volume of ground water to the well. It is also expected that the majority of ground water yielded by AW-9 is from the silt which is present from approximately 9.5 to 17.5 fbg. Additionally, all wells on the site were installed in accordance with standard practices where the screened interval extends above the saturated zone. The construction of AW-9 would allow for separate-phase hydrocarbons, if present, to accumulate in the well. There is no evidence of separate-phase hydrocarbons at the site. The screened interval intercepts`the gravel fill material within the formerly excavated area. Soil samples collected from the capillary fringe area (e.g., 9.5 to 10.0 fbg) contained no detectable concentrations of TPH-G, and 1.8 ppm TPH-D. The sample collected at 11.5 to 12.0 fbg contained 480 and 510 TPH-G and TPH-D, respectively; these are not the concentrations we would expect to see when dealing with a "separate-phase site." If separate-phase hydrocarbons were present, this would be observed in the field when static water levels are measured (prior to purging) with an electronic interface probe. First Bullet Item, Paragraph 4 Alton Geoscience does not agree with the statement that benzene is detected in most of the on- site monitoring wells. Historically, low concentrations of-benzene have been detected in onsite ground water monitoring wells AW-9, AW-10, and AW-12 (from below detection limits to 15 ppb). Benzene has not been detected in onsite monitoring wells AW-1, AW-2, AW-3 (0.5 ppb when first installed), AW-4, or AW-11. A more accurate statement would be that benzene has not been detected in most of the onsite monitoring wells. Geomatrix further states that "Benzene is the most mobile of the hydrocarbon constituents and the most easily removed from the separate phase hydrocarbons held in the silt unit." If it is assumed that Geomatrix' statement is true, the detection of relatively low concentrations of benzene in ground water would support 6