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1 <br /> ARCADIS GERAGHTY&MILLER <br /> 6 5 3 1 Dissolved Oxygen <br /> ' Dissolved oxygen O <br /> xyg {D )was measured at the Site using field instrumentation during the <br /> ' February 5, 1999 monitoring event DO concentrations were lowest at wells closest to the <br /> plume (well U-3) and higher in wells away from the center of the plume DO concentrations <br /> ' measured using field instrumentation during the February 5, 1999 monitoring event are shown �4 <br /> on the Well Monitoring/Sampling Field Data Sheets in Appendix D The distribution of <br /> ' dissolved oxygen, depletion of oxygen near the center of the hydrocarbon mass and lesser <br /> depletion away from the center, is consistent with the occurrence of biodegradation of the <br />' groundwater plume <br /> 6 5 3 2 Nitrates <br /> When nitrate is used as an electron acceptor in microbial degradation of a petroleum <br /> hydrocarbon plume, the concentration of nitrates in groundwater is expected to decrease <br />' Nitrate concentrations in samples collected during the February 5, 1999 monitoring event are <br />' summarized in Table 3 of Appendix D Figure 6-5 shows a isoconcentratnon plot of nitrates in <br /> groundwater at the Site on February 5, 1999 Cross gradient concentrations of nitrate in the <br />' northerly direction are greater than those found in the plume center Under conditions in which <br /> there is a relatively high background nitrate concentration, as indicated by data from other <br />' sources in the Stockton area, one would expect a nitrate contour map to indicate a low nitrate <br /> area which coincides with the higher concentrations in the comparable hydrocarbon plume map <br />' (as shown in Figures 6-1 and 6-2) The data shown by this plot of Site nitrate concentrations <br /> are not conclusive with respect to utilization of nitrate at the Site as an electron acceptor The <br />' 1, 10, and 100 ug/I, contours do not close on the southern side of the plume Once again, it <br /> cannot be ruled out that the explanation for this Ices with the potential release of petroleum <br />' hydrocarbons from the former service station, shown on the 1972 Sanborn maps, that used to <br /> operate south, and generally upgradient, across E N ner Avenue from the subject Site While <br /> 44 <br />