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S E C O R <br /> • 2003 Feasibility Study Report and Site Assessment Work Plan <br /> July 18, 2003 <br /> Page 10 <br /> 4.1.1 Mobility <br /> Overall migration of contaminants, especially dissolved LNAPLs, is mainly determined from aquifer <br /> properties including hydraulic gradient, groundwater flow direction, porosity, and aquifer hydraulic <br /> conductivity. These aquifer properties remain relatively constant regardless of the chemicals <br /> introduced into the aquifer system. Generally, dissolved phase contaminants will flow in the direction <br /> of groundwater flow at a rate determined by the aquifer hydraulic conductivity, gradient, and, <br /> porosity. <br /> Rate of contaminant migration can also be affected by properties of a specific chemical combined <br /> with the aquifer properties. These properties, which vary from contaminant to contaminant, include <br /> advection, dispersion, and degradation. <br /> Additional properties that can vary the rate of contaminant migration are properties that are affected <br /> by the nature or properties of the specific chemical combined with the aquifer properties. These <br /> properties, which vary from contaminant to contaminant, include advection, dispersion, and <br /> degradation. <br /> Advection describes the transport of miscible constituents at the same velocity as the groundwater. <br /> Hydrodynamic dispersion refers to the spreading of a miscible contaminant over a region greater <br /> than that predicted solely from the advective mechanism. Hydrodynamic dispersion is comprised of <br /> molecular diffusion, which is a result of variations in concentrations, and mechanical dispersion, <br /> which is a result of deviations of the actual groundwater velocity from the average velocity. <br /> Molecular diffusion affects are deemed negligible when compared to mechanical dispersion effects <br /> for aquifers having highly conductive materials. The molecular diffusion was assumed to be <br /> negligible for the purpose of our model. <br /> Transverse dispersivity values are typically found to be an order of magnitude less than the <br /> longitudinal dispersivity values, while vertical dispersivity values are typically found to be two orders <br /> of magnitude lower than the longitudinal dispersivity values. These ratios are again based on <br /> empirical relationships available in literature for similar lithologies. The dispersivity values and ratios <br /> were further modified during the initial model calibration process. This was essential to obtain a <br /> good correlation between observed and simulated contaminant values. <br /> Linear sorption and first order irreversible rate reactions (radioactive decay or biodegradation) also <br /> affect the transport of contaminants in groundwater. Sorption is the process that causes the slower <br /> migration of contaminant with respect to groundwater. Sorption can be defined as the mass transfer <br /> process between the constituents dissolved in the solution phase and the constituents adsorbed on <br /> the solid phase. The partitioning between these two phases is called the distribution coefficient (Kd). <br /> Sorption is generally incorporated into a transport model through the use of the retardation factor, <br /> which is related to the distribution coefficient by the following formula: <br /> R = 1 + Kd (p/n) <br /> where, - <br /> R retardation factor for a water quality constituent <br /> Kd = distribution coefficient <br /> I:\Chevron\1001621\REPORTS\2003\2003FS\FS_2003 revD.doc <br />