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OH KLEINFELDER i 4 FATE AND TRANSPORT MODELING 1 ' Based on the groundwater sample analytical results from the October 2004 sampling event and iso-concentration contour Interpretations, fate and transport models for ' benzene and TPH-G have been prepared as requested in SJCEHD correspondence dated September 24, 2004 The models have been presented in Appendices C and D, respectively 41 MODEL CALCULATIONS iPSC evaluated groundwater models for benzene and TPH-G as contaminants-of- concern using an ASTM Risk Based Corrective Action (RBCA) approach (ASTM E- 1739-95) This model provides an indication of the risk for benzene and TPH-G to jeopardize potential receptors, particularly through the groundwater ingestion exposure ' route The RBCA groundwater model calculation predicts a contaminant concentration at a distance X along the centerline of a groundwater plume emanating from a vertical planar source in the aquifer This model accounts for three-dimensional dispersion and ibiodegradation The model allows for typical default values and for site-specific data, such as source concentration, source width, source thickness, hydraulic conductivity, iand groundwater gradient Parameters in the equation include X = Distance from the planar source to the location of concern, along the centerline of the groundwater plume (In a down gradient direction) i CX = The concentration of the contaminant at a distance X from the source Cs = The concentration of the contaminant at the source ax,y,Z =Dispersivity in the x, y, and z directions iU = Specific discharge (flow velocity) using hydraulic conductivity (K), total soil porosity (6T) and hydraulic gradient (1) = First order degradation constant of the contaminant SW = Width of groundwater source In the y direction iSp = Depth of groundwater source in the z direction C23-484517-BOOISAC5R540 Page 10 of 14 August 11, 2005 Copyright 2005 Kieinfelder,Inc