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field investigation data or estimated based on industry standards AT123D was used to determine receptor point concentrations of BTEX constituents at an imaginary ' well at the southwestern property line downgradient of the plume (Figure 6) The default chemical parameters, obtained from scientific research literature, included in the DSS databases were used. Although AT123D allows a lumped first order decay rate, decay rates were not specified for this simulation A point source at MW-9, the well at the leading edge of the plume, was assumed using the highest concentrations of each contaminant from the fourth quarter 1995 groundwater monitoring event An instantaneous chemical release was assumed because the source has been ' removed and the mass of the chemical present in the saturated zone could be estimated based on field data ' The input parameters for the Stockton Service Center AT123D model are presented in Appendix D These parameters were agreed upon by the SJCPHS. The ' input parameters used for the site are considered to be conservative representations of the site conditions and the contaminant properties Further documentation on the model and input parameters is presented in Appendix D 3.3 Results ' The contaminant fate and transport model indicated that the BTEX constituents will not reach California maximum contaminant levels (MCLS) within 100 years at ' a receptor point downgradient of the plume at the property boundary. Only benzene reaches the receptor point within the 100 year simulation time The maximum ' concentration of benzene at this receptor point is 0 023 micrograms per liter (at 100 years) Toluene, ethylbenzene, and xylene do not reach the receptor point within 100 years The model indicates that the BTEX constituents will not migrate offsite within ' 100 years, except for very low concentrations of benzene The receptor point used for the model was an imaginary well: the shallow aquifer in which BTEX has been found is not used for drinking water. The closest drinking water well is approximately 0 5 mile from the site, but this well is screened in a deeper aquifer (approximately 200 feet), and may no longer be in use Because bioremediation was not considered and conservative input parameters were used, the model results are very conservative. The contaminant fate and transport model results support the conclusion that the Stockton Service Center does not pose a risk to human health or the environment ' 96R7 East Say Municipal Utility District 40424700 fcr 15 Final Closure Report