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CAP Addendum:Former Fuvg Station, 7500 West Eleventh Street, Tracy, G Page 13 Redox Potential = -0.0 11(TPHd+TPHg) +83.97 (EQUATION 1) That relationship indicates that when the Redox Potential is zero, the sum of the TPHd and TPHg dissolved in the groundwater would be 7,634 µg/L. Because, in general terms, when the Redox Potential of the groundwater becomes positive, natural attenuation by aerobic processes are initiated and there is a large increase in the rate at which those processes will remediate the groundwater compared to the anaerobic or negative Redox condition. The reasonableness of the above estimate of the maximum concentrations of components of fuel.hydrocarbons that will permit aerobic processes to be activated can be checked by considering the highest contaminant concentrations that have been measured in groundwater in wells at the Navarra Site, which, from the evaluation studies of natural attenuation processes at the.site, is known to be remediating under the action of aerobic contaminant attenuation processes. On March 9, 2005, at the time when the natural attenuation evaluation studies were performed, as recorded in Table 2, the highest concentration of dissolved components of fuel hydrocarbons (i.e., TPHd and TPHg) in groundwater in any well where aerobic conditions prevailed was 2,450 µg/L in Monitoring Well MW-3. That is less than the estimated limiting value of 7,634 µg/L for aerobic processes of natural attenuation to be active. However, in May 2000, the total concentration of dissolved components of fuel hydrocarbons in the groundwater in Monitoring Well MW-3 was 13,800 gg/L, with both the gasoline and diesel concentrations below the most conservative estimates for their respective saturation concentrations. The value of 13,800 µg/L of dissolved components of fuel hydrocarbons is also greater than the estimated limiting value of 7,634 µg/L required for aerobic processes to be active. Despite this, it has been clearly demonstrated that aerobic natural attenuation processes were active in groundwater in the area around Monitoring Well MW-3 in May 2000, as evidenced by the rapid decline in the concentrations of components of fuel hydrocarbons in the groundwater that occurred in the following months (The San Joaquin Company Inc. 2006). The above finding illustrates the inherent uncertainties involved in making predictions of the reduction in contaminant loading required to switch attenuation of dissolved components of fuel hydrocarbons from anaerobic to the much faster-acting aerobic condition. However, if for the purpose of making a general estimate, we assume that the concentration of TPHd + TPHg in groundwater in an area of the site must be reduced to approximately 13,000 µg/L, the question, "What is the optimum contaminant mass that must be removed to enable natural attenuation to complete the remedial action in a reasonable time period?" can be reduced to: "What amount of the contaminant mass must be removed to reduce the total concentration of dissolved diesel and gasoline in groundwater in the area of the Navarra Site around Monitoring Well MW-7 to approximately 13,000 gg/L?"