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Mr.Marty Hartzell Febntary 16, 2001 Page 7 of 8 Al SB4. Field reading using a FID generally confirmed these observations. ' No detectable concentrations of TPH& TPHd, BTEX and MTBE were found in soil samples collected from borings SB2 or SB4. The sample collected from boring SB3 at 40.5 feet bgs contained only TPHg, ethylbenzene and xylenes at 1.4 ppm, 0.005 ppm, and 0.038 ppm. Analytical results from boring SBI indicated that no significant impapt to the soil appears to exceed a depth of 40 feet bgs. Only trace amounts of TPHg was detected at 45 feet bgs(1.4 ppm), but very low levels of xylenes persisted to 60 feet bgs in boring SB 1. No detectable concentrations of TPHd, TPHg, BTEX, oxygenated gasoline additives, or lead scavengers were detected in the groundwater samples collected from borings SB 1 and SB4. ' 6.0 CONCLUSIONS AND RECOMMENDATIONS 61 Conclusions • The lateral and vertical extent of soil contamination has been defined and is generally limited to the interval between the depths of approximately 15 feet and 40 feet in the immediate vicinity of ' the former gasoline UST. • Preliminary estimates of the mass of residual contamination remaining in soil are approximately M25 kg (55 lbs) diesel, 2,068 kg (4,550 lbs) gasoline, 5.4 kg (12 lbs) benzene, 55.6 kg (122 lbs) toluene, 51.8 kg(114 lbs)ethylbenzene, and 256 kg(563 lbs)xylenes. • Ground water has not been impacted by the release of gasoline from the former UST. The majority of significant contamination appears to bound within the silt unit at a depth of approximately 40 feet bgs, and little migration appears to have occurred as indicated by the trace levels present in the sand lens at 45 feet bgs. Furthermore, soils surrounding and below the sand lens consist of very stiff to hard silts and clays, which are expected to provide an effective confining barrier between the contaminants and shallow aquifer. • Based upon the absence of groundwater contamination beneath the site and the presence of a ' confining layer at a depth of approximately 38-43 feet bgs, which apparently prevents further downward migration of contaminants, the potential threat to groundwater posed by the residual soil contamination at the site is very small. 6.2 Recommendations ' Although the potential threat to groundwater posed by residual soil contamination is very small, in order to eliminate uncertainties related to the potential threat to groundwater Ground Zero recommends conducting a limited feasibility study to determine if soil vapor extraction would be an ' effective remediation alternative in reducing residual contamination in the soil and thus reducing the potential threat to groundwater. The feasibility study would include the installation of one or more vapor extraction test wells, performing a short term vapor extraction test using an internal ' a&UNgrou=X o=Wcs\stmupLdoe