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SECTION 5.0 RESULTS AND DISCUSSIONS <br /> without nitrate, sulfate, dissolved oxygen and redox data for the entire site over time, it is unclear <br /> whether microbes are present and utilizing electron acceptors to naturally degrade petroleum <br /> hydrocarbons at Stockton Yard However, the historical decrease in TPH-g, BTEX, and MTBE <br /> concentrations in groundwater, as presented in Table 3-3 and shown for TPH-g and benzene in <br /> Figure 5-3, provide evidence that natural attenuation of these compounds in groundwater has <br /> occurred and is 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) estimating the mass of contaminants in soil both during previous <br /> investigations (included in this report) and currently (accounting for petroleum hydrocarbons <br /> partitioning in soil vapor and groundwater), and (3) assessing whether hydrocarbons oxidizing <br /> microbes and nutrients are present in the subsurface to naturally degrade petroleum <br /> hydrocarbons The scope of work was limited to the estimation of the mass of petroleum <br /> hydrocarbons in soil based on previous investigations <br /> 1 <br /> S 4PROJWlo4ol00041SYYARMMAINWAROSA_DOC 5-5 URS Greiner Woodward Clyde <br />