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biodegradation is not considered in any of the model runs, all three model scenarios represent very conservative predictions of the potential impacts to the shallow aquifer 5 3 HYDROLOGIC PARAMETERS 1 Hydrologic and soil chemical input and output parameters are shown in Table 9 Hydrologic ' parameters include • Infiltration rate, and • Depth to groundwater ' In the Stockton area, the average annual infiltration was assumed to be 20 percent of the annual rainfall of 15 inches for unpaved conditions However, because the site is capped with asphalt, an ' infiltration rate of 2 percent of precipitation was assumed As described above, three depths to groundwater, 35 feet, 54 feet, and 70 feet bgs were considered In all three cases, a groundwater ' gradient of 0 002 feet per foot and a hydraulic conductivity of 105 cm/sec was assumed This value of hydraulic conductivity was obtained from the literature for the lower end of the scale for the sandy silts and silty sands found at the site A low value of hydraulic conductivity was used to be conservative ' 5 4 SOIL CHEMICAL PARAMETERS ' Soil chemical parameters used in the model include the following • Organic carbon distribution coefficient (K,,,,,), • Henry's constant (KH), • The aqueous solubility, • The free air diffusion coefficient, • Dry bulk density, • Total porosity, • Volumetric water content, and • Organic carbon fraction An estimate of 0 5 percent soil organic carbon content was used The volumetric water content, solubility, free air diffusion coefficient and Henry's constant were obtained from the literature Soil properties were estimated based on typical values for sandy silts to silty sands ' SAC116 07 12 AUGUST 1994 DAMES & MOORE