ARCHIVED REPORTS_2003 FEASABILITY STUDY REPORT AND SITE ASSESSMENT WORK PLAN

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S E C O R 01 2003 Feasibility Study Report and Site Assessment Work Plan July 18, 2003 Page 13 2. Calibrating the chemical reaction terms by modifying first the Koc values and then the half-life of the COC, keeping the source concentration and the dispersivity values constant. 3. Modifying the dispersivity values after achieving initial calibration of the source concentration and the chemical reaction values. 4. Modifying the chemical reaction terms subsequent to obtaining the initial corrected source concentration and dispersivity values. The AT123D simulations are described in the following sections. 4.4 AT123D Simulations The following section describes parameters utilized for each AT123D simulation. The sections are separated into the model setup modules within AT123D. 4.4.1 Simulation Parameters For each COC, two simulation parameters were modeled. The first simulation was conducted during a 5 year time period to calibrate the model to known site conditions. The second simulation was conducted for a 30 year time period to evaluate future migration of COC. A third simulation was conducted for additional years to verify COC migration extent. 4.4.2 Simulation Controls and Aquifer/Source Definition After selecting the chemical of concern, the type and duration of contaminant, or waste, release is estimated, in units of kilograms per hour (kg/hr). Since this term is not a field measured term, SECOR estimated the amount of contaminant that was released by varying the amount until the source area in the model exhibited contaminant concentrations that were similar to the contaminant concentrations observed at the site by the existing groundwater monitoring well network. In addition, during the five-year simulation, SECOR conservatively assumed that the source contaminant would continuously enter the aquifer system at a constant rate during this time span. For the 30-year simulation, SECOR conservatively assumed that the source contaminant would continuously enter the aquifer system at a constant rate for a minimum of 10 years. The reason for assuming this is to provide model results that would be a reasonably conservative assessment on contaminant fate and transport. Given this, the following contaminant release rates were used: ■ a contaminant release rate of 0.5 kq/hr was applied to the model for MtBE with a source area equaling 200 meters cubed (m ); • a concentration of 10 kg/hr was applied to the model for TPHd with a source area equaling 600 m3; and • a concentration of 0.1 kg/hr was applied to the model for TPHg with a source area equaling 200 m3. For each simulation, the aquifer was assumed to be of infinite in thickness and a real extent. I:\Chevron\1001621\REPORTS\2003\2003FS\FS 2003 revD.doc