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The slope of the trendline for TPHg,in well MW-2 is -0.0019 and representsthe first-order degradation constant for TPHg in well MW-2-equal to.-0.001.9..day 1. Based on the degradation constant, the half-life of TPHg can be calculated as follows: t 0.5=ln(0:5)/slope where: t 0.5 = 'half--life of TPHg[day] ln(0.5) = naturalflog of 0.5(equals-0.693) slope = first-order degradation constant[day- Using day Using the equationpresented above,-the TPHg half-life in well MW-4 is 365 days. This'graphical method used to determine site-specific degradation,rate and half-life is consistent with the approach presented in "Regression Techniques and Analytical. Solutions to Demonstrate Intrinsic ` Bioremediation. In:Intrinsic.Bioremediation"(Buscheck and Alcantar,. 1.995)and "Calculation and Use. of First-Order Rate Constants for Monitored Natural. Attenuation Studies". (EPA/540/S-02/500, November 2002). Using the graphical method discussed above, the TPHg and TPHd concentration data for wells MW-2, MW-13,.and VW-2 were plotted.to determine site-specific degradation rates. The plots are included as Figures 9 and 10. Based on the natural logarithmic:plots, the calculated first-order degradation rate constants (in day") are as follows: TPHg 'TPHd MW-2 0.0019 -0.0007 MW-13 0.0014 -0.0003 VW-2 -0.0009 -0.0005 Average -0:.0014 -OA005 The corresponding calculated half-lifes(in days)are As.follows: TPHg _ TPHd MW=2 365 990 MW-13 495 2,310 VW-2 770 1,386 Average 495 . 1,386 Initial Concentrations Initial chemical concentrations in groundwater for TPHg,and TPHd were approximated based on the most recent groundwater monitoring.. data' which were.collected in January 2013 (Figures 4 and 5). The initial model concentrations are shown on Figures 1 i and 12. Stockton Yard Maintenance Station,TO No.1$` Caltrans Contract No.06A1895,EA No.43-910206 Geocon Project No.S9800-01-OIB -7- September 30,2013