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UO field investigation data or estimated based on industry standards. In every case, the input parameters assumed the most conservative values in order to yield the highest factor of safety for the model. 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 9). 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 LO of the site conditions and the contaminant properties. Further documentation on the model and input parameters is presented in Appendix D. 3.4 Results The contaminant fate and transport model indicated that the BTEX constituents #6. will not reach California maximum contaminant levels (MCLS) within 100 years at a receptor point downgradient of the plume at the property boundary. Benzene, AW 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. 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 ti. in a deeper aquifer (approximately 200 feet), and may no longer be in use. Because conservative input parameters were used, the model results are very conservative. L,. The contaminant fate and transport model results support the conclusion that the Stockton Service Center does not pose a significant risk to human health or the environment. 9687 East Bay Municipal Utility District 40429700.tcr 13 Final Closure Report i--/ Lr