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5.0 CONCLUSIONS AND RECOMMENDATIONS The conclusions of this site characterization investigation include the following: I 1. Soil sample analytical results indicate that the main portion of the site soil petroleum hydrocarbon contamination has been contained in the near surface silt and fine rained sand is Y g i to a depth of not more than 16.5 feet bgs. The vast majority of contaminated soil was contained in the soil between 6.5 and 11.5 feet bgs. 2. Total original contaminant mass of approximately 400 to 700 cubic yards has been identified as having been present inplace prior to UST removal and subsequent overexcavation. Soil sample analytical results indicate the average concentration of this soil mass was as follow: BTEX, MTBE and oxygenates all below analytical method detection limits; total petroleum hydrocarbon as diesel fuel (100 to 9100 mg/kg). These concentrations are based on analysis of soil sample analytical results from the UST removal and 1999 soil boring results. 1 3. Current remaining petroleum hydrocarbon contaminated soil mass is estimated to have an average concentration of 50 to 100 mg/kg of TPH-d. This soil contamination does not extend i below 16.5,feet bgs. 4, Groundwater beneath the site has a trace amount ofTPH-d at a maximum concentration of 150 ug/l. The groundwater plume is small in extent(<1,000 square feet)and appears to attenuate rapidly in the estimated down-gradient flow direction. Average TPH-d concentrations for all three grab sample locations is 71 ug/l. This TPH-d water concentration is below the MCLS for the basin. No volatile organic compounds are present in site groundwater. Recommendations include: 1. Site closure. Environmenta1lSTE\PARSCR09i02008.wpd 30 r 9 II i i.