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O Hydrogeology. Currently groundwater occurs at approximately 25 5 fbg beneath the site Measured depths to groundwater in the site wells have ranged historically from 15.1 to 29 5 fbg, and groundwater elevations have ranged historically from 7 2 to 20 3 feet above nisl Shallow groundwater flow beneath the subject site is variable but generally is toward the southwest at a gradient ranging from 0 001 to 0 003 A general description of the VLEACH model, model setup, and the fate and transport modeling results are presented below VLEACH Model Description VLEACH is a one dunensionat finite difference model used to assess potential groundwater impacts and volatilization of volatile organic constituents (VOCs) from unsaturated soils The model assumes a one-layer homogeneous profile and a constant recharge rate at the surface In the subsurface, the model assumes constant flow and transport of VOCs through soil by the n following transport mechanisms -sorption, volatilization,-diffusion to the atmosphere, and VOC leaching to groundwater The model does not take into account VOC degradation and dispersion, which are fundamental fate and transport mechanisms for gasoline constituents Because of these assumptions, the model output results can be quite conservative VLEACH Model Setup The modelmg results are presented in Attachment A As shown, Profile 1 presents the Model Settings, Profile Structure, Chemical, and graphical representation of the model output for MTBE(Figure 1) The model settings and results are presented below Model Settings- The Model Settings are shown in Table 1- Case Settings and Table 2 - Initial Conditions For the Case Settings table the simulation timestep or model recalculation interval was set at 01 years, and the simulation length was set at 100 years The cell number was set at 15, which equals the depth to groundwater We used 15 fbg as the depth to groundwater because this is the most representative value to use based on the soil sample collection date of August 1996 The representative area is not shown in the model settings (but is part of setup) is shown on Figure 2 and is approximately 0 004 acre or 176 ft2 The surface water recharge rate is 0 17 feet per year This value is approximately 10% of the annual rainfall for the area and assumes that this is the rate of water entering a paved surface and moving through the unsaturated zone 0323 2