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is Fr <br /> Mr Harlin Knoll <br /> November 29, 1995 (9347-0085) <br /> • Comparison of the analytical results from this investigation and the December <br /> 1988 investigation, indicate that the adsorbed-phase petroleum hydrocarbons <br /> detected in the soil samples collected near the locations of samples TS-2, PL-5, <br /> and PL-6 have decreased over 90 percent The apparent petroleum hydrocarbon <br /> attenuation may be attributed to the combined effect of natural biodegradation, <br /> volatilization, dispersion, and diffusion processes in the subsurface soil The <br /> residual petroleum hydrocarbons should continue to be degraded through natural <br /> processes <br /> • Review of the analytical results indicates that the extent of the adsorbed-phase <br /> petroleum hydrocarbons are generally limited to the areas near western fuel <br /> dispenser islands and the former UST basin <br /> • It does not appear that the petroleum hydrocarbons detected in the subsurface soil <br /> pose a threat to ground water that warrants further investigation or engineered <br /> remediation This is based on (1) the implied depth to ground water <br /> (approximately 68 feet bg), (2) the rate of natural attenuation (over 90 percent <br /> reduction in petroleum hydrocarbon concentrations since 1988), (3) the maximum <br /> concentration of petroleum hydrocarbons detected in the samples collected in <br /> . September 1995 (100 ppm TPH-G), and (4) the low permeability soil types <br /> encountered during drilling (silt with occasionally silty sand unit) <br /> 9347-0085 ARP <br />