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SECTIONFOUR Up®sum Assasm OR Another conservative assumption in the modeling is that the source is assumed to be infirnte and ' constant The amount of chemical that reaches the groundwater is calculated in the ASTM RBCA program by using the steady-state ratio between the concentration of an organic constituent in soil pore water and the source concentration of the affected soil mass This ratio, called the soil leachate partition factor (Ksw), is used to represent the release of soil constituents to leachate percolating through the affected soil zone and is written as follows - Ksw[(mg l L—H2 O)l(mg/kg—soil)) = p, /B,. +k,p, +HO,. where ' Ksry= Soil leachate partition coefficient -10 p, = Soil bulk density(g-soil/cm3 soul) B„. = Volumetnc water content in vadose zone soil(cm3 H2O%m3-sail) k, = Soil water sorption coefficient(gH2O/g-soil) Bp, = Volumetric air content in vadose zone soils(cm3-air/cm3-soil) A leachate dilution factor (LDF) is calculated to account for the dilution of organics as leachate from the overlying affected soil zone mixes with groundwater in the underlying water-bearing unit The ' LDF divided by the Ksw represents the steady-state ratio between the concentration of an organic chemical in the groundwater zone and the source concentration on the overlying affected soil To ' estimate the LDF, a simple box model is used to estimate mass dilution within a mixing zone m the water-bearing unit directly beneath the affected soil mass ' LDF = 1+VZ,,,SB,,, l IW where Vg,,, =Groundwater Darcy velocity(cm/s) 89W =Groundwater mixing zone thickness(cm) ' I =Infiltration rate(cm/year) W =Width of source area parallel to wind, or groundwater flow direction(cm) ' The above equations assume that deep percolation through affected soil reaches the groundwater regardless of soil thickness or permeability ® T 119971973009NAWOCKTC)MREPOM-04 REM DOCe&FEB-99V9 MMNA%SNA 4-7