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Tn3advvell8Rdb 3.0 SOIL AND GROUNDWATER CONDITIONS Groundwater is generally encountered in wells at the Site at approximately 15 to 18 feet below ground surface (bgs) The Site is underlain by silts and clays with some shallow silty sand layers Groundwater elevations advanced in January 2004, borings at the Site generally stabilized at 17 to 18 bgs The release that occurred at the former Tank No 7 location has been identified as being ' responsible for the groundwater contamination currently present at the site Two monitoring wells, MW-3, located adjacent to the former UST location, and down gradient well MW-9 have significant concentrations of petroleum hydrocarbons Up to 23 inches of free product was encountered in well MW-3 in 1992 (AGE, 1999) From 1994 to approximately 1997, there was a local rise in groundwater of at least 10 feet From November 1995 to February 1996, the groundwater rose above the top of the recorded well screen elevation in MW-3, and the measurable free product thickness decreased and finally Iceased appearing in MW-3 during this same period (AGE, 2003) If free product was present during this period, it is not expected that it would have been detected in MW-3, as the top of the well screen terminated below the groundwater elevation Since groundwater analyses began in 1996 at MW-3, the concentrations of total petroleum hydrocarbon as gasoline (TPHg) have not shown a significant change, ranging between 300,000 mg/kg and 110,000 mg/kg In spite of several years of in-situ remediation technologies (air sparging, vapor extraction and ozone spargmg) operating at the Site, the average groundwater concentration has been approximatcly 180,000 micrograms per liter(pg/L) (Treadwell & Rollo, 2004) During groundwater grab sampling in January 2004, bonngs 16B-6b and 16B-8 (Figure 3) had similar TPHg and benzene concentrations to MW-3, and a hydrocarbon sheen was observed on groundwater in these bonngs 25971021 DJS 3 18 November 2004