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Geologceal T Imic s Inc Page 14 ' Additional Site Characterization Report Project No 662 2 February 20, 2004 5.0 CONCLUSIONS AND RECOMMENDATIONS Based on our interpretation of the data collected over the course of this subsurface investigation, GTI have reached several conclusions These conclusions are based on the premise that the data we considered, although incomplete, are representative of actual site ' conditions We acknowledge that there may be undiscovered conditions, which would upon their consideration, change our interpretation and thus our conclusions Our recommendations are based on our knowledge of site conditions, and on the state and limitations of subsurface investigative technology ' 5.1 Conclusions • The CPT investigation confirmed a previous conclusion that soil contamination is limited to the vicinity of the former UST excavation The highest concentration of MTBE in soil was detected in a clay unit in CPT-4 @ 35 feet bgs— 1 2 mg/kg ' • The MTBE is part of a submerged plume migrating below the water table and as such was not detected in shallow groundwater • The distribution of MTBE suggests that the sandy units at 38 —48 feet bgs are acting @� as a preferential pathway for contaminant migration The MTBE plume is not widespread but does extends off site as evidenced by the water sample from CPT-7 at 47 feet bgs 100 ug/l MTBE • There is a core plume containing TPH-G beneath the site, but no significant BTEX is present This suggests that the MTBE and TPH-G are from an old release and 1 resistant to further degradation This also suggests that no gasoline is currently leaking from the existing system due to the lack of BTEX 5.2 Recommendations GTI makes the following recommendations + Additional monitoring wells should be installed in the down gradient direction in order to further define the extent of the groundwater plume The wells should be ' screened to intercept the sand layers at 38 --48 feet bgs • Perform a dual phase aquifer pumping test to determine if soil vapor extraction is ' feasible at the site and to determine aquifer pumping characteristics • Extraction well EW-1 was installed to perform an aquifer pumping test But unfortunately the screened interval of this well (37 — 50 feet bgs) will not allow ' vapor extraction to address shallow soils at the site GTI recommends installing a second extraction well to address the shallower residual soil contamination beneath q the former UST excavation