ARCHIVED REPORTS_2003 FEASABILITY STUDY REPORT AND SITE ASSESSMENT WORK PLAN

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S E C O R • 2003 Feasibility Study Report and Site Assessment Work Plan July 18, 2003 Page 12 Solubility, vapor pressure, organic carbon partitioning coefficient, and density affect the mobility of the contaminants. COCs detected in soil and groundwater at the site generally have relatively high solubility in water, high vapor pressure, and low organic carbon partition coefficient (Ko,). Additionally, COCs at the site would be expected to easily leach from the unsaturated zone and move with groundwater in the water-bearing zone. The persistence of the contaminants is related to their degradability. In the subsurface, the primary mode of degradation would be biodegradation by indigenous microorganisms. CDCs at the Site have been shown to readily degrade in the subsurface by anaerobic and aerobic mechanisms. 4.2 Model Selection Fate and transport models that describe the migration of constituents in groundwater must have the ability to simulate the on-site conditions and long-term chemical fate in groundwater, determine the distance traveled by a constituent in groundwater at any given time, and predict the constituent concentration distributions in groundwater for future time based on historical data trends. In addition, the models should also be validated and also be readily acceptable by the regulatory agencies. The RISKPROTM system, which is a management tool designed to help users perform exposure assessments, was used to develop the model. For the groundwater simulations, the Analytical Transient 1-, 2-, 3-Dimensional Model AT123D (G.T. Yeh, 1981) was used to compute and predict the transient spread of the contaminant plume through the groundwater aquifer, estimate the dissolved concentration of the chemical in groundwater resulting from a mass release over a source area, and assess the potential down-gradient impact and extent of the three CDCs in groundwater. 4.3 Modeling Strategy The first step in developing the model is to select the COC. Selection of the chemicals of concern is based on analytical results and the physical properties of the chemicals found at the Site. This review includes physical properties like solubility in water, solubility in organics, maximum concentrations of the chemicals, and the potential health impact based on the MCL's for the underlying aquifer. Based on all these factors, MtBE, TPHg, and TPHd, were selected as the chemicals that best represented the site-specific conditions for the "worst-case" scenario. The next step in performing any fate and transport analysis is to develop a conceptual model. This is done by initially developing a representative site stratigraphy model. Boring log and aquifer test data were integrated to determine the physical parameters of the aquifer material. The model was calibrated for each of the selected COC consistent with the procedures presented in ASTM standard D5880-95 (ASTM, 1996) and D5981-96 (ASTM, 1996b). Model transport parameters conditions were adjusted in a systematic manner until an acceptable fit was obtained between the model solution and the target concentration data. This initial calibration was conducted according to the following approach: 1. Calibrating the source concentration to a final source concentration, without modifying the dispersivity and chemical reaction terms. 11Chevron\1001621\REPORTS\2003\2003FS\FS 2003 revD.doc