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CAP Addendum:Former FueQ Station, 7500 West Eleventh Street, Tracy, CQ Page 10 the processes of natural attenuation that are currently acting to dissipate the concentrations of analytes of concern in groundwater. These issues are discussed below. 2.3 Optimal Amount of Contaminant Mass to be Removed To assess the optimal mass of components of fuel hydrocarbons that should be removed from the subsurface by the proposed remediation technology, consideration must be given to processes of natural attenuation that are active in the soil and groundwater in the area affected by LNAPL. 2.3.1 Aerobic and Anaerobic Processes of Natural Attenuation SJC has completed an evaluation of natural attenuation processes active in the subsurface beneath the Navarra Site (The San Joaquin Company Inc 2006). That study showed that aerobic natural attenuation processes were clearly active in groundwater at the locations of all groundwater monitoring wells (see Figure 1 for locations) except in the area around Monitoring Wells MW-7 and MWFP-1. Figures 5 and 6 are plots of the distributions of the natural logarithms of concentrations of BTEX and dissolved oxygen and the natural logarithms of concentrations of BTEX: and reduction oxidation potential (Redox Potential), respectively, along Hydrostratigraphic Section C-C' (see Figure 1 for location and Figure 3 for hydrostratigraphy), which section passes through, or close to, both Monitoring Wells MW-7 and MWFP-1. As cited in Table 4, the Redox Potential of the groundwater in those two wells as measured in February 2005 was -246.7 mV and -142.4 mV, respectively. Negative values of Redox Potential are indicative of anaerobic conditions in the groundwater. As is also cited in Table 4, the chemical oxygen demand of the dissolved material in groundwater in Monitoring Wells MW-7 and MWFP-1 are both high at 22 mg/L and 50 mg/L, respectively. High chemical oxygen demand is indicative of an environment that is unlikely to support aerobic natural attenuation processes. As a general rule of thumb, 5.0% dissolved oxygen in groundwater affected by fuel hydrocarbons is considered the minimum required to support aerobic processes of natural attenuation. In February 2005, there was only 3.2% dissolved oxygen in the groundwater in Monitoring Well MW-7. That, taken together with the values other geochemical parameters measured during the natural attenuation study, reveals that, at that time, anaerobic conditions prevailed in groundwater in the area around that well. At that time, the concentration of total petroleum hydrocarbons in the groundwater at that well (i.e., approximately the sum of the concentrations of TPHd and TPHg) that are recorded in Table 2 was 33,000 p.g/L and the apparent thickness of LNAPL floating on the surface of the groundwater as recorded in Table 1 was 0.02 ft. Evidence that dominantly anaerobic conditions were prevailing in Monitoring Well MWFP-1 in February 2005 is less strong. Although, as was noted above, the Redox