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31 March 2006 AGE-NC Project No. 98-0471 Page 10 of 13 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 Ground Water transport model. Based on soil profile descriptions of a dominantly silt to silty sand aquifer separated by intervals of silty clay,default soil geotechnical parameters were selected by the RISC program.Utilizing ground- water infiltration rate equations of Connor et al (1997), an averaged infiltration rate of 1.06 inches/year was calculated for silt and organic soils,based on a City of Stockton average rainfall of 17.3 inches/year. 8.1.1. 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 (cross-gradient direction), and thickness of 20 feet (6.1 meters), 15 feet (4.6 meters), and 16 feet (4.9 meters), respectively. Plume dimensions were calculated to reflect an approximate plume volume, excluding all excavated material, of approximately 136 cubic meters.Ground water fluctuation was calculated to be approximately 1 foot (0.3 meters),and silty sand was assumed to be the general soil type.Default geotechnical parameters selected by the RISC program, soil plume dimensions, and other input parameters are included in Appendix N.The average concentrations of benzene,toluene,ethylbenzene,xylenes,and MTBE in soil were calculated to be 3.19 mg/kg, 10.31 mg/kg, 4.14 mg/kg, 29.74 mg/kg, and 6.72 mg/kg, respectively(Table 8). Calculations and ground water concentration output summaries for BTEX compounds and MTBE are included in Appendix N-1. The Saturated Soil To Ground Water model predicts that the benzene source concentration will be non-detectable at<0.005 mg/kg after 1 year;the toluene source concentration will be non-detectable at <0.005 mg/kg after 2 years; the ethylbenzene source concentration will be non-detectable at <0.005 mg/kg after 2 years;the xylenes source concentration will be non-detectable at<0.005 mg/kg after 2 years;and the MTBE source concentration will be non-detectable at<0.05 mg/kg after 1 year (Table 9; Appendix N-2). The liquid phase concentrations for benzene, toluene, ethylbenzene, xylenes, and MTBE over a 100-year period would range from an initial 9,850µg/1 benzene to non-detectable levels(<0.5 µg/1) in 2 years; from an initial 8,110µg/1 toluene to non-detectable levels(<0.5 µg/1)in 2 years; from an initial 907µg/1 ethylbenzene to non-detectable levels(<0.5µg/1)in 2 years;from an initial 7,630µg/1 xylenes to non-detectable levels(<0.6µg/1)in 2 years;and from an initial 33,500µg/1 MTBE to non- Advanced GeoEnvironmental,Inc