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' 0 0 <br /> 10 7k adwell&Rollo <br /> ' 3.0 SOIL AND GROUNDWATER CONDITIONS <br /> 1 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 advanced in January 2004, borings at the Site generally <br /> stabilized at 17 to 18 bgs. <br /> The release that occurred at the former Tank No. 7 location has been identified as being <br /> 1 responsible for the groundwater contamination currently present at the site. Two monitoring <br /> wells, MW-3, located adjacent to the former UST location, and down gradient well MW-9 have <br /> 1 significant concentrations of petroleum hydrocarbons. <br /> ' Up to 23 inches of free product was encountered in well MW-3 in 1992 (AGE, 1999). From <br /> 1994 to approximately 1997, there was a local rise in groundwater of at least 10 feet. From <br /> NNovember 1995 to February 1996, the groundwater rose above the top of the recorded well <br /> screen elevation in MW-3, and the measurable free product thickness decreased and finally <br /> 1 ceased appearing in MW-3 during this same period(AGE, 2003). If free product was present <br /> during this period, it is not expected that it would have been detected in MW-3, as the top of the <br /> 1 well screen terminated below the groundwater elevation. <br /> 1 Since groundwater analyses began in 1996 at MW-3, the concentrations of total petroleum <br /> hydrocarbon as gasoline (TPHg) have not shown a significant change, ranging between <br /> 1 300,000 mg/kg and 110,000 mg/kg. In spite of several years of in-situ remediation technologies <br /> (air sparging, vapor extraction and ozone sparging) operating at the Site, the average <br /> groundwater concentration has been approximately 180,000 micrograms per liter(µg/L) <br /> (Treadwell&Rollo, 2004). During groundwater grab sampling in January 2004, borings 16B-6b <br /> 1 and I B-8(Figure 3)had similar TPHg and benzene concentrations to MW-3, and a <br /> hydrocarbon sheen was observed on groundwater in these borings. <br /> 1r <br /> 1 25971021.DJS 3 18 November 2004 <br />