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• 31 August 2005 AGE-NC Project No 96-0232 Page 23 of 30 and Health Risk modeling are presented in Appendix N 71 FATE AND TRANSPORT Utilizing RISC, AGE conducted a 100-year fate and transport study to analyze the potential impact of contaminated soil and ground water to the CSA municipal water well No 2, located approximately 700 feet down-gradient of the site(Figure 3), and to indoor air in the large mini-mart building at the site (Figure 20) Since the potentiometnc surface of first-encountered ground water fluctuates seasonally, from as high as approximately 6 feet bsg between March and May to as low as 20 feet bsg between August and December,AGE analyzed the fate of contaminant concentrations in soil beneath the UST location utilizing a`Saturated Sod To Ground Water'contaminant transport model, in which the contaminants are leached and transported from soil by seasonally-fluctuating ground water In this model,the mass of chemicals contained in the portion of the source above the water table(if any)is assumed to be leached due to infiltration The mass in the portion of the source below the water table (if any) is assumed to be leached horizontally with the ground water flow AGE also analyzed fate and transport of current contaminant concentrations in ground water, utilizing a `Dissolved Source To Ground Water' transport model Based on soil-profile description of a dominantly silty sand to medium sand aquifer, separated by intervals of sandy clays and sandy silts,default geotechnical parameters were selected by the RISC program (Appendix O) Based on a City of Stockton average rainfall of 17 3 inches/year, AGE calculated an average ground-water infiltration rate of 0 7 inch/year, utilizing rate equations of Connor and others (1997), for ground water infiltration from surface grade to 6 feet bsg through dominantly clay-and silt type soil beneath an asphalt cover Additional model parameters,including dimensions of impacted soil and ground water sources, are provided in Appendix O The analysis was conducted for MTBE in soil and ground water, utilizing the more conservative MTBE concentrations determined from the 95%upper confidence limit(UCL)of the mean Tables 11 and 13 contain the following statistics mean concentrations and associated standard deviations, 95%UCL of the mean,confidence interval and the number of samples utilized for averaging AGE also tested how sensitive the models were to increasing hydraulic conductivities, MTBE concentrations, subsurface depths, and more permeable soil types (Tables 12, 14 and 15) 7 1 1 Saturated Soil To Ground Water Model The input parameters utilized for the typical "saturated soil to ground water" model include soil plume length (down-gradient direction), width and thickness of 115 feet (38 meters), 210 feet (70 meters) and 12 feet(4 meters),respectively, a ground-water fluctuation of approximately 9 feet (3 Advanced GeoEnvironmental,Inc