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A Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, C4. .Page 34 at Monitoring Well MW-3 and the oxygen content of the groundwater begins to rise again as the BTEX concentrations decline with distance eastward from the axis of the plume. Unfortunately, the complete relationship, which would be expected to show a full } recovery of dissolved oxygen in groundwater to the same percentage as is the case at the western periphery, cannot be illustrated on Figure 32. The complete relationship between BTEX and dissolved oxygen along section B-B' would require geochemical data from a point beyond the eastern perimeter of the plume, which is estimated to be located between Monitoring Well MW-4 and the center line of Chrisman Road. SJC originally designed a monitoring well array that included a well along the alignment of B-B' that was to be installed along the eastern side of Chrisman Road and would have provided the missing geochemical data (The San Joaquin Company Inc. 1999), but the unlicensed SJCEHD staff that oversaw the project at that time would not permit its installation (San Joaquin County Public Health Services 2000). Figure 33 shows the relationship between BTEX and dissolved concentrations along the f line of section C-C'. Those plots also show that there is a significant depression in the dissolved oxygen in the groundwater in the area around Monitoring Well MW-7, where there are relatively high concentrations of fuel hydrocarbons. h�1 The relationship between BTEX concentrations within the plume and dissolved oxygen along longitudinal and cross sections of the affected area of groundwater provide persuasive secondary evidence that processes of natural attenuation are vigorously active at the 7500 West Eleventh Street site. 7.2.1.2 Oxidation Reduction Potential ;< Figures 34-36 are companions to Figures 31-33, but instead of showing the relationships between concentrations of BTEX and dissolved oxygen along lines A-A', B-B' and C-C', they illustrate the relationships between the concentrations of BTEX compounds and redox potential along those section lines. In areas where the redox potential is lower than some 500 mV, it is possible that some of the natural attenuation processes are reducing in nature. When the redox potential falls to a negative number, it is clear that all aerobic _ processes cease and the breakdown of the components of fuel hydrocarbons is solely due to anaerobic reactions. Examination of the redox potential curves on Figures 34-36 shows a well-developed inverse relationship between that parameter and the concentration of F.<. BTEX in groundwater. On both Figures 34 and 36, it is clear that in the area around Monitoring Wells MW-7 and MWFP-1, where redox potential falls into a negative range, anaerobic rather than aerobic conditions prevail. Supporting geochemical evidence for �:. that finding is discussed 7.2.2 below. 7.2.2 Evidence for Anaerobic Respiration i As has been discussed above, anaerobic bioremediation can occur in oxygen-starved environments. Such low oxygen conditions are often found at the sites of fuel hydrocarbon releases in portions of the plume where components of fuel hydrocarbons are at high concentrations. Direct indicators of anaerobic bioremediation are elevated i sic i� r.