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 14 4.4.3 Soil and Waste Properties Soil and chemical (waste) parameters were assigned values based on literary data available for the selected compounds. The bulk density of the soil was assumed to be 1380 kg/m3 with a porosity of 0.30, which lies within the range of values that are specified for sandy soils (reference). The hydraulic gradient for the site is assumed to be 0.04 ft/ft, based on an average of hydraulic gradients collected at the site during previous and ongoing groundwater monitoring activities. Hydraulic conductivity (K) of the aquifer was assumed to be 0.2 meter per hour (m/hr), based on the average K value determined from site aquifer testing data (SECOR, 2003d). Since each COC was modeled for the same aquifer, the above described soil and aquifer properties were held constant for each simulation. Chemical transport retardation values were estimated using the bulk density, porosity and fractional organic carbon content (foc) values for the soil, and Koc values for the each of the CDCs. The decay constant for MtBE (.00092/hr) was determined based on literary data available on half-life values (Howard etaL, 1991). The decay constant for TPHd and TPHg was based on decay constants for constituents within both products. During the calibration process, decay constants of 0.00032/hr and 0.00008/hr were assigned for TPHg and TPHd, respectively. The distribution coefficient (Kd) was determined by published values for Koc for each COC and assuming a foc value of 0.1% (or 0.001) and a porosity value of 0.3 were assumed for the fill soil. SECOR obtained an MtBE Koc value of 5.258 liters per kilogram (L/kg) from the Risk Assessment Information System (RATS) web page (http://risk.Isd.ornl._gov/rap hp.shtml). SECOR obtained a range of Koc values for TPHd and TPHg from the The Association for Environmental Health and Sciences (AEHS) web page (www.aehs.com). Since a range of log Koc values were given for TPHd and TPHg, the Kd value was varied during the calibration process to best simulate known site conditions. The Kd value was not varied outside a range that would not include the range of Koc values obtained for TPHd and TPHg. Table 1 provides the values of and the references for the different soil and chemical parameters that were used in the model. Values for the longitudinal, transverse, and vertical dispersivity were also varied during the calibration process in order to best simulate the plume configuration observed at the site. For MtBE, longitudinal, transverse, and vertical dispersivity values included 20, 5, and 0.1 meters, respectively. For TPHd, longitudinal, transverse, and vertical dispersivity values included 40, 4, and 0.1, respectively. And for TPHg, longitudinal, transverse, and vertical dispersivity values included 5, 2, and 0.1, respectively. The higher longitudinal dispersivity ensures that the contaminant front migrates predominantly in the longitudinal direction. 4.4.4 X, Y, and Z Coordinates AT123D provides the user to define grid locations in the X, Y, and Z directions. The user is limited to define only 15, 10, and 10 grids points for the X, Y, and Z directions, respectively. SECOR utilized existing groundwater monitoring wells at the site to set grid locations. By basing the grid locations on the existing well network, SECOR could utilize the existing groundwater monitoring data to calibrate the model and estimate values for longitudinal, transverse, and vertical dispersivity. For TPHg and MtBE, the same coordinate system was utilized. The coordinate system was modified for TPHd to I:\Chevron\1001621\REPORTS\2003\2003FS\FS__2003_revD.doc