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 11 p = bulk density of the aquifer sediments n = effective porosity of the aquifer sediments Organic compounds are hydrophobic and are sorbed by weak forces. The sorption of these compounds is related to the amount of organic carbon in the aquifer materials. Kd can be determined from the organic carbon content by the following equation: Kd =fo, x Ko, where, fogy = fractional organic carbon concentration in the aquifer sediments Kos = distribution coefficient of the organic compound in water and 100 percent fraction organic carbon This relationship implies that the retardation factor is dependant upon the percentage of organic carbon contained within the aquifer. A first order irreversible process that would represent radioactive decay or biodegradation was included for the initial solute transport run of the model. This phenomenon can be attributed to the . occurrence of source decay with respect to groundwater impacts. The decay rate constant (X) for a compound, which is usually given in terms of the half-life of that compound, can be estimated from the following equation: X= In(2)/t112 where t1,2 is the half-life of the degradable material (the time required for the concentration toto decrease to one-half of its original value). Initial radioactive decay values were selected based on available published data (Howard, etal., 1991). A single degradation value for each COC was assigned. An initial decay constant for MtBE was based on the range of half-life values of MtBE obtained from literature research. Initial decay constants for TPHg and TPHd where based on known half-life values of selected TPHg and TPHd constituents (i.e., benzene, toluene, styrene, fluorene, naphthalene, etc.). These initial values were modified, as necessary, during the calibration processes of the transport model. Decay constants were not varied outside the range of published half-life values. 4.1.2 Toxicity Toxicity of contaminants in groundwater is based on the persistence of the contaminant and the completed pathway of the contaminant to a receptor. Generally, the fate and transport of contaminants at a site are evaluated by considering the physical and chemical interactions of the contaminants within the environment. Various parameters, including mobility, persistence, and abundance, are considered in the evaluation. Of the parameters, mobility and persistence are of primary importance. Mobility represents the potential for a chemical to migrate along a given pathway. Persistence is a measure of how long a chemical will remain in the environment. 11Chevron\1001621\REPORTS\2003\2003FS\FS_2003_revD.doc