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02 June 2004 AGE-NC Project No 99-0645 Page 5 of 5 SITE CONCEPTUAL CONCLUSIONS Based on the data collected from the site, AGE concludes ■ The sand units occurred at depths of 15 to 30 feet bsg and 75 or 80 feet to 80 or 85 feet bsg The deepest sand unit previously encountered may actually be two thinner units Groundwater was encountered at approximately 96 feet bsg Ground water flow direction at the site was northwest The decrease of ground water elevation of approximately 4 feet between July 2002 and October 2003 may be due to seasonal fluctuation • TPH-g was detected in the soil boring sample collected north of the site (B6') Low concentrations of MTBE were detected in the soil sample to a depth of 55 feet bsg in the same boring Benzene concentrations were detected in the two deepest soil samples collected from the northern boring, indicating the northern migration of only benzene, at a depth of 60 feet to 70 feet bsg or the presence of another source off the northern edge of the site With no detections of benzene in the upper 55 feet of boring B676, only low detections of benzene in the former UST soil boring 131, all less than 0 1 mg/kg, and the lack of detectable hydrocarbons in the soil boring MW-1R below 50 feet bsg, the source of the off-site benzene detected in the further most reach of boring B6' appears to be from another source than the UST release However, soil vapor extraction on the site will likely effectively mitigate the . detections off-site TPH-g was not detected in the soil boring sample collected at the west edge of the site(VW3) Only low concentrations of MTBE were detected at 30 and 40 feet bsg in boring VW3 TPH-g was not detected in the soil boring sample collected at the southern edge of the site(B7) Only one detection of MTBE was in the soil, from 30 feet bsg in boring B7 TPH-g was only detected in the upper most soil sample from boring VW1A, at the east edge of the site MTBE was detected in samples from boring B7 from 40 feet to 60 feet bsg The lateral extent of adsorbed MTBE may extend below the eastern boundary of the site Soil vapor extraction on the site will likely effectively mitigate the MTBE off-site • Generally, the highest concentrations of MTBE were detected within a 45 foot thick interval, generally occurring between 15 feet and 60 feet The vertical extent of the MTBE-impacted soil was limited to less than 70 feet bsg • MTBE was detected in one ground water sample(MW-1) at a concentration of 120 µg/1 This concentration does exceed the maximum contaminant level for MTBE in drinking water • Based upon effective vacuum potential of 0 1 inches of water, the calculated effective radius of influence at the site for vapor wells screened from 15 feet to 40 feet bsg will be approximately 40 feet The calculated effective radius of influence at the site for vapor wells screened from 40 feet to 70 feet bsg or greater will be approximately 30 feet • TPH-g,TPH-d and BTEX compounds were not detected in the three water samples collected • Advanced GeoEnvsronmenW