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j <br /> Treadwell&Rollo <br /> 3.0 SITE GROUNDWATER CONDITIONS <br /> Groundwater is generally encountered in wells at the Site at approximately 15 to 18 feet below <br /> ground surface (bgs) The Site is underlain by silts and clays with some shallow silty sand <br /> layers Groundwater elevations in recent borings at the Site generally stabilized at 17 to 18 bgs <br /> The release that occurred at the former Tank No 7 location has been identified as being <br /> responsible for the groundwater contamination currently present at the site Two monitoring <br /> wells, MW-3, located adjacent to the UST location, and down gradient well MW-9 have <br /> significant concentrations of petroleum hydrocarbons <br /> Up to 23 inches of free product was encountered in well MW-3 in 1992 From 1994 to <br /> approximately 1997 there was a local rise in groundwater of at least 10 feet From October 1995 <br /> to February 1996 the groundwater rose to above the recorded well screen elevation in MW-3, <br /> and the measured fiee product decreased and finally ceased appearing in MW-3 during this same <br /> period If free product is still present, it is not expected that it would be detected in MW-3, as <br /> the top of the well screen currently terminates 5 to 7 feet below the groundwater elevation <br /> Since groundwater analyses began in MW-3 during 1996, the total petroleum hydrocarbon as <br /> gasoline (TPHg) concentrations have not shown a significant change, and have averaged <br /> approximately 180,000 micrograms per liter(µg/L) This is in spite of the fact that, for several <br /> years, in-situ remedial technologies (air sparging, vapor extraction and ozone spargmg) have <br /> operated at the site <br /> ' This data suggest that there is there is a continuing source of gasoline contamination at the <br /> former UST No 7 location, and that free product may still be present If free product is present <br /> in groundwater, or high concentrations of TPHg are still present in soil at the Former UST No 7 <br /> location, than continued in-situ remediation may not effectively mitigate the groundwater <br /> ' contamination <br /> 3 <br /> 25971014 DOD 7 April 2004 <br />