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v <br /> ikTc <br /> A S S O C I A T E S I N C <br /> depletion of oxygen from aerobic activities. Concentrations of Eh are generally lower within the <br /> plume than outside the plume indicating bioremediation is occurring within the contaminant <br /> plume. Dissolved oxygen, carbon dioxide, and Eh appear to be good indicator parameters for <br /> intrinsic bioremediation at this site. <br /> Based upon geotechnical and geochemical data collected from groundwater and/or soil underlying <br /> the project site, it appears that bioremediation is taking place in the contaminant plume beneath <br /> the site. Biodegradation appears to be continuing in an aerobic and anaerobic environment. A <br /> review of groundwater chemical data suggests the contaminant plume is stable since it appears <br /> that petroleum hydrocarbon concentrations are decreasing in the wells. <br /> Additional evidence of bioremediation occurring in groundwater beneath the site is found in <br /> the soil vapor analytical data. Carbon dioxide concentrations are higher in vapor samples from <br /> borings inside the plume than those outside the plume, suggesting bioremediation of the <br /> �. petroleum hydrocarbons in the capillary fringe. Also, methane was detected in the soil vapor <br /> sample from boring SB2, which is inside the plume, suggesting this is a byproduct of <br /> bioremediation of the petroleum hydrocarbons and floating product in the capillary fringe. <br /> bw <br /> The results of chemical analyses are presented in Tables 1 through 3. Geochemical trends are <br /> summarized in Table 5. <br /> 4.2 Further Evaluation <br /> As previously discussed, it is now generally recognized that a major factor responsible for the <br /> attenuation of mass reduction of BTEX in groundwater plumes is hydrocarbon biodegradation by <br /> ` indigenous microorganisms in aquifer material. Our objective is to apply well-known regression <br /> techniques and analytical solutions to estimate the contribution of advection, dispersion, and <br /> sorption, and biodecay to the overall attenuation of petroleum hydrocarbons. These calculations <br /> yield an apparent biodecay rate based on field data. This biodecay rate is a significant portion of <br /> the overall attenuation in stable, dissolved hydrocarbon plumes. <br /> �. 4.2.1 Calculation of Biodegradation Decay Rates Based on Chemical Analytical Data <br /> Biodegradation as a function of time and distance are best described by the first order exponential <br /> functions shown below(Buscheck and Alcantar, 1995). <br /> rr <br /> C(t) = Cie-(`t) (Eequation 1) <br /> C(z) = Coes") (Equation 2) <br /> where C(t) is the concentration of a contaminant at a specific point at time, t; C(x) is the <br /> concentration of a contaminant at a specific time at point, x; C; is the initial concentration at t=0; <br /> CQ is the concentration at the source; k is the first order attenuation rate; and v is the advective <br /> groundwater velocity. <br /> w:\95122Veports\ReWeA for NFAdoc 11 <br /> r <br />