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%WO *"we <br /> Model Input Parameters <br /> Source Concentration (Co). Co determined from soil concentration according to the <br /> relationship <br /> Co = Cs/Kd <br /> where: Cs = soil concentration (milligrams per kilogram [mg/kg]) <br /> Kd= soil-to-water partition coefficient <br /> Kd was calculated as follows: <br /> Kd = Koc x foc (Freeze & Cherry, 1979) <br /> where: Koc = organic carbon-to-water partition coefficient(milliliter per gram [mllg]) <br /> foc = soil organic carbon content (unitless) <br /> Values for Koc are from Attachment A, Region IX preliminary remediation goals (PRGs) <br /> first half 1995 (USEPA, Region IX, February 1995). Values for foc were assumed to be <br /> 1 percent (0.01). ' <br /> The concentrations in the source area are presented in Tables A-1 through A-4. <br /> Distance (L). This distance (I8 meters) was taken to be the distance from the bottom of <br /> the interval below where chemicals were detected in the soils from boring B-1 (i.e., <br /> 11.5 feet BGS) to the depth to groundwater (i.e., 72 feet BGS). <br /> Depth (Z) and Width (X) of Plume in the Saturated Zone. The chemical results <br /> from boring MW-I show that chemicals were detected in soil collected from i I to <br /> 11.5 feet below ground surface (BGS), and no compounds were detected in the interval <br /> 15.5 to 16 feet BGS. The value for Z was therefore assumed to be about 3 feet (i.e., I <br /> meter). The width of the impacted area was not determined at this site. To determine the <br /> effect this parameter has on the modeling outcome, therefore, we assumed a range of <br /> widths (i.e., 1 meter, 5 meters, 10 meters, and 50 meters). This is discussed further <br /> below. <br /> Dispersion Coefficient (Dt). Hydrodynamic dispersion (i.e., the spreading of a plume <br /> out from the source) is the combined result of two dispersive processes: mechanical <br /> dispersion and molecular diffusion. In most aquifers, the rate of molecular diffusion is <br /> considered negligible compared with that of mechanical dispersion. This is especially true <br /> in coarse-grained aquifer material such as that found at this site. For this reason, <br /> molecular diffusion will not be considered in this evaluation. For any given aquifer, <br /> mechanical dispersion is a function of groundwater velocity. Dispersion perpendicular to <br /> the flow path is defined as follows: <br /> JASECT%PJA2074001 0.1 BR-9511jt:2 A-2 <br />