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with our model are higher than expected future groundwater contamination levels are likely to be To conduct the modeling in a conservative manner we assumed no migration retardation and applied a full recharge rate equivalent to the annual Stockton precipitation rate In reality, the subsurface at each of the two locations modeled would be subjected to very little infiltration due to • Man-made features -- Location I is beneath a 300 x 100 foot building Location II is beneath areas covered by concrete and pavement • Natural features -- within the vadose zone is a 5-foot thick clay layer between the contaminated soil and the shallow water bearing zone In remaining with our conservative approach, we also omitted the effects that are associated with natural degradation of these types of contaminants A discussion of model parameter impact sensitivities is contained in Appendix A VLEACH output data is generated as annual contaminant mass loading to the groundwater The Federal EPA SNARL and CAL EPA MCL's for drinking water are provided in ppb concentrations In order to compare the modeling output to regulatory standards, a unit conversion must be done. The procedure for accomplishing this is described in Appendix B 9.2 Model Results 9.2.1 Xylenes and Ethylbenrene 9.2.1.1 Location 1, Basement Area According to CAL EPA, the MCL for total xylenes is 1750 ppb and 680 ppb for ethylbenzene (Marshack, 1991) Even with our conservative assumptions (i e , applying full annual recharge rate, omitting the influence of the clay layer barrier in the soil column, and omitting the natural decay and degradation of the contaminants), modeling shows that the concentrations of these compounds in the groundwater will not exceed their respective MCLS (Table 5) For example, the maximum concentration of total xylenes in groundwater will occur in 70 years and ethylbenzene in 60 years (Table 6) The predicted maximums are 3 0 x 104 ppb and 168 x 10' ppb, respectively These concentrations are 6 to 7 orders below the MZ/04-06-95/JST/95-0089 15