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AW�s�� Is Working To Restore Nature concentrations upon initial contact with the groundwater. Natural dilution and diffusion properties would be expected to reduce the dissolved concentration fairly rapidly According to the above, residual soil contamination at the site should not be capable of impacting groundwater to a significant extent CONTAMINANT TRANSPORT The estimated maximum groundwater velocity for the subject site is calculated to be approximately 3 ft/yr, based upon the published maximum estimated hydraulic conductivity in similar soils (1 gpd/ft2), the estimated effective porosity of the soil (10%), and measured hydraulic gradient beneath the site (0 006). The rate of migration of dissolved contaminants in the groundwater is generally less than the groundwater velocity due to naturally occurring processes. The three general categories of processes affecting subsurface transport of petroleum hydrocarbons are hydrodynamic, abiotic (physiochemical) and biotic Each of these processes in turn are dependent upon the physical properties of the subsurface and the physiochemical and biologic properties of petroleum hydrocarbons. Hydrodynamic Processes The formation parameters controlling hydrodynamic processes at the site are incompletely known, but an understanding of these processes and the parameters which affect them give a general sense of how the contaminant plume beneath the site might be affected 40 Hydrodynamic processes impact contaminant transport by affecting the rate and direction(s) of groundwater flow and include advection, dispersion and preferential flow Advection is the movement of contaminants within the groundwater system and is dependent upon groundwater velocity Dispersion causes spreading of the contaminant over a much larger area than advection alone would produce and, consequently, a dilution of the contaminant away from the source. Preferential flow would tend to increase contaminant mobility in one or more direction. There is no evidence of preferential flow pathways in the vicinity of the contaminant source at the subject site It can be concluded that the dominant hydrodynamic processes affecting contaminant transport beneath the site are advection and dispersion (which tends to dilute contaminant concentrations as the plume migrates away from the source) Abiotic Processes The transport and fate of petroleum hydrocarbons at the site will be significantly affected by abiotic processes (sorption) Low solubility, non -polar organic molecules, such as petroleum hydrocarbons, have high distribution coefficients (high sorption potential) The primary factor controlling sorption (and therefore retardation) is the organic carbon content of the sediments in the water bearing zone The organic matter in Jacktone Clay is approximately 2-5% (USDA, 1992) which corresponds to 1 16-2 9% organic carbon content The retardation factor (Rd) is defined as the velocity of groundwater divided by the velocity of the solute, and can be estimated using the soil distribution coefficient (Kd) of the contaminant and the bulk AMk density (6) and porosity (e) of the soil as follows (Hounslow, 1990, Freeze and Cherry, 1979) 1500541R4D2-994 DOC 3