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22 October 1996 <br /> AGE-NC Project No. 95-0100 <br /> Page S of 9 <br /> ' 3.4. DSS MODELING OUTPUT. <br /> The model predicted that no benzene, ethyl benzene, toluene and xylene would impact the ground <br /> ' water at the site. A one-hundred year estimation of hydrocarbon impact to ground water was <br /> established for the model. Appendix G includes SESOIL input data and output graphs. <br /> ' <br /> 3.5. AT123D MODELING OUTPUT <br /> ' The results of the AT]23D model indicted that a significant increase of the ground water,elevation <br /> at the site, would cause the integrity of the ground water to be compromised. If the ground water <br /> elevation were to increase approximately 20 feet to a depth of 35 feet bsg, a significant portion of <br /> ' the hydrocarbons adsorbed to soil on the site would be exposed to ground water and could be <br /> dissolved into the water. Concentrations of the initially benzene-impacted ground water were <br /> t estimated at 25,000 ppb. After 6.5 years the dissolved benzene concentrations would be reduced to <br /> levels below the California Department of Health Services maximum contaminant level of 1.0 ppb. <br /> The model also predicted that other BTE&X compounds would dissolve-into the elevated ground <br /> water. Concentrations of the other BTE&X compounds would also decline over a period of time. The <br /> increased ground water elevation model is unlikely to occur, based on a survey of historical ground <br /> water elevations for the vicinity of the site. Appendix H includes AT1231) input data and output <br /> ' graphs. <br /> 4.0. CONCLUSIONS <br /> The following conclusions were drawn from the assessment of the site: <br /> • Based on laboratory analysis of soil samples collected during this and previous investigations <br /> at the site, it appears that hydrocarbon-impacted soil is distributed through the upper vadose <br /> ' zone, beginning at a depth of approximately 5 feet bsg and extending to a depth of 35 feet <br /> bsg. <br /> • The results of the theoretical fate and transport DSS model indicate that the hydrocarbon <br />' constituents of concern; benzene toluene <br /> ethyl benzene and total xylene <br /> yimpacting the soil, <br /> are not Iikely.to migrate to the ground water underlying the site. <br /> 5.0. LIMITATIONS . <br /> Our professional services were performed using that degree.of care and skill ordinarily exercised by <br />