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. SECTION 5.0 RESULTS AND DISCUSSIONS <br /> nitrate, sulfate, D O and redox data for the entire site over time it is unclear whether microbes <br /> are present and utilizing electron acceptors to naturally degrade petroleum hydrocarbons at Shop <br /> 10 However, the historical decrease in TPH-g, BTEX, and MTBE concentrations in <br /> groundwater, as presented in Table 3-2 and shown for TPH-g and benzene in Figure 5-4, provide <br /> evidence that natural attenuation of these compounds in groundwater has occurred and is <br /> occurring at the site <br /> Natural attenuation (or mass reduction) of petroleum hydrocarbons in soil may not be evaluated <br /> without (1) collecting additional petroleum hydrocarbon data in the vadose zone and adsorbed to <br /> soil beneath the water table, (2) accurately depicting the mass of contaminants in soil during <br /> previous investigations and currently(accounting for petroleum hydrocarbons partitioning in soil <br /> vapor and groundwater), and (3) assessing whether hydrocarbons oxidizing microbes and <br /> nutrients are present in the subsurface to naturally degrade petroleum hydrocarbons The scope of <br /> work was limited to the estimation of the mass of petroleum hydrocarbons in soil based on <br /> previous investigations <br /> 1 <br /> 1 <br /> 1 <br /> S IPROM1040100041SHOPIC4SHOP10SA.DOC 5-5 UBS Greiner Woodward Clyde <br />