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30 <br /> Figure 6-3 shows the interpreted isoconcentration contour map of the distribution of TPHg as <br /> depicted for the Site by Applied Geosystems in May of 1990 Comparison of Figure 6-3 with <br /> Figure 6-1 illustrates that the TPHg plume front has not substantially moved in rune years <br /> Note that Figure 6-3 concentrations are shown m units of mg/L, while Figure 6-1 <br /> concentrations are shown in units of ug/L Similarly, Figure 6-4 reproduces the interpreted <br /> nsoconcentratnon contour map of the distribution of benzene as depicted for the Site by Applied <br /> Geosystems in May of 1990 Comparing Figure 6-4 with Figure 6-2 illustrates that the <br /> ' benzene plume front has also not substantially moved over the 9-year monitoring time interval <br /> ' 6.5.3 Electron Acceptor Utilization <br /> ' Biodegradation of petroleum hydrocarbons by microorganisms ultimately results in the <br /> production of carbon dioxide, methane, and water During biodegradation, microorganisms <br /> ' transform available nutrients into forms useful for energy and cell reproduction by facilitating <br /> the transfer of electrons from donors to acceptors This results in oxidation of the electron <br /> donor (natural organic material and petroleum hydrocarbons) and reduction of an electron <br /> acceptor The more important electron acceptors in groundwater include dissolved oxygen, <br /> ' nitrate, iron (III), sulfate, and carbon dioxide Biodegradation causes measurable changes in <br /> groundwater geocherrustry During aerobic respiration, oxygen is reduced to water, and <br /> ' dissolved oxygen concentrations decrease Once the dissolved oxygen is depleted, microbes <br /> will increasing utilize less energy-efficient alternative electron acceptors rutrate is reduced to <br /> NOi,N20,NO,NW, or N2i iron(III)is reduced to iron(I1), sulfate is reduced to H2S, CO2 is <br /> ' reduced by methanogenic bacteria and CI-14 is produced The following discussions evaluate <br /> spatial trends in the February 4, 1999 monitoring data to ascertain the presence of microbial <br /> ' biodegradation processes <br /> 6 5 3 9 Dissolved Oxygen <br /> ' Dissolved oxygen O was measured at the Site during the February 4 199 <br /> xS'g � ) g �' 9 <br /> monitoring event DO concentrations were lowest at wells closest to the plume(wells MW-2, <br /> 1 <br /> MW-1, MW-3, MW-6, MW-10) and highest m wells away from the center of the plume DO <br />