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Layer 3(sand and sandLa er 3(sand and sand c <br /> • Thickness=323 feet <br /> • Top elevation=-105 feet <br /> • Bottom elevation=-428 feet <br /> • Total porosity=35% <br /> • Effective porosity=20% <br /> • Hydraulic conductivity=5x10-3 cm/sec <br /> Although the public supply water well is used for emergency backup and is reportedly only turned on for <br /> periodic sampling,we conservatively modeled the well in Layer 3 with a screened interval of-187 to-404 <br /> feet and at a constant pumping rate of 2,000 gallons per minute (gpm) under transient conditions. <br /> The location of the public supply well in the center of the model domain is shown on Figure 7. <br /> Contaminant Transport <br /> Contaminant transport of TPHg (as benzene) and TPHd(as naphthalene) was simulated using MT3DMS <br /> incorporated in the Visual Modflow Pro software package. MT3DMS is a newer version of the MT3D <br /> model,which is a solute transport model for simulation of advection,dispersion and chemical reactions of <br /> dissolved constituents in groundwater systems. <br /> Distribution Coefficients <br /> Chemical-specific organic carbon partition coefficients for benzene (59 cubic centimeter per gram [cm3/g]) <br /> and naphthalene 10,200 cm3/g) and an assumed value of 1% fraction of organic carbon in soil (American <br /> Society for Testing and Materials 1739-95 default value) were used to calculate chemical-specific <br /> distribution coefficients. The calculated distribution coefficients simulated were 5.9x10"L/mg for TPHg(as <br /> benzene)and 1.2x10-5 L/mg for TPHd(as naphthalene). <br /> Degradation Rates <br /> Geocon constructed natural logarithmic plots for concentrations of TPHg and TPHd for wells MW-2, <br /> MW-13, and VW-2. This plotting method provides for a straight trendline that can be used to predict <br /> approximate concentrations at points in time and establish site-specific degradation rates. <br /> As shown on natural logarithmic plot for TPHg in well MW-2(see Figure 9),the equation of the trendline <br /> is: <br /> y=-0.0019x+79.855 <br /> where: <br /> y = natural log of TPHg concentration <br /> -0.0019 = trendline slope [day'] <br /> x = Date(since start date) <br /> 79.855 = y-intercept based on start date <br /> Stockton Yard Maintenance Station,TO No.1B Caltrans Contract No.06A1895,EA No.43-910206 <br /> Geocon Project No.S9800-01-01B -6- September 30,2013 <br />