ARCHIVED REPORTS XR0011755 - Laserfiche WebLink

Home Browse Search

1� Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, CA. Page 37 1 relatively high and where,based on the evidence developed in Section 7.2.2.1 and 7.2.2.2 above, anaerobic reactions are attenuating the concentrations of fuel hydrocarbons. E As is shown on Figure 42, there is also a correlation between manganese and BTEX concentrations in groundwater along section B-B'. This suggests that, although the ;+ dominant natural attenuation processes occurring in that area of the plume are aerobic, some anaerobic processes may also be active there. 7.2.2.4 Sulfate 1 Figures 43 and 44 show the correlations between sulfate in groundwater and the concentrations of BTEX compounds along sections C-C' and B-B', respectively. Decreased sulfate concentrations can indicate that anaerobic biodegradation processes are using sulfate as an electron receptor for reduction of petroleum hydrocarbons. It is clear from inspection of Figure 43 that those processes are active in the vicinity of Monitoring F =� Well MWFP-1 on section C-C'. 'i As was the case for manganese, there is also a correlation between BTEX and sulfate concentrations in groundwater along section B-B'. This finding tends to corroborate the hypothesis that at least some portion of the natural attenuation processes active in the groundwater around section line B-B' are anaerobic, even though the principal degradation processes active in that area are aerobic. ` 7.2.2.5 Methane Elevated concentrations of methane in groundwater can indicate active anaerobic `==� degradation of fuel hydrocarbons b processes that use carbon dioxide as an electron � y y acceptor. Again, as is illustrated in Figures 45 and 46, concentration of methane in samples recovered from the wells at the Navarra Site are significantly elevated in the area where anaerobic bioremediation has been demonstrated to be active by the evidence presented in Sections 7.2.2.1 through 7.2.2.4 above. 7.2.2.6 Carbon Dioxide Figures 47 and 48 that show plots of carbon dioxide and BTEX concentrations in groundwater along sections B-B' and C-C', respectively, illustrate that there is a distinct ' correlation between those geochemical parameters. Elevated concentrations of carbon dioxide in groundwater may indicate that aerobic biodegradation processes are active. Depleted levels of carbon dioxide in groundwater can indicate that methanogenesis is occurring where anaerobic degradation processes are active. However, carbon dioxide data must be reviewed carefully because it can be influenced by many complex geochemical interactions. For those reasons, SJC will not attempt to interpret Figures 47 and 48 except to point out that there are clear changes in carbon dioxide concentrations in groundwater within the plume of contamination relative to those in groundwater beyond the periphery of the plume. sic