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CAP Addendum:Former FueQg Station, 7500 West Eleventh Street, Tracy, L Page 11 Potential was low at -142.4 mV and the chemical oxygen demand was relatively high at 50 mg/L, the dissolved oxygen in the groundwater in that well measured 6%, which is generally considered sufficient to sustain aerobic processes of natural attenuation. At that time, the total concentration of fuel hydrocarbons in Monitoring Well MWFP-1 was 12,700 µg/L but, as was always the case throughout the period when that well was checked for the presence of floating product, it contained no LNAPL. Based on those findings, SJC concludes that the geochemistry of the groundwater in Monitoring Well MWFP-1 is such that, although marginal, aerobic processes of natural attenuation are being minimally sustained in that area of the site. Although anaerobic natural attenuation processes are capable of remediating the subsurface beneath a site affected by petroleum hydrocarbons, they act much more slowly than aerobic processes. This is primarily because aerobic conditions are required to sustain soil biota, which voraciously digest petroleum hydrocarbons to reduce them to harmless carbon dioxide and water. The natural attenuation evaluation study performed for the Navarra Site (The San Joaquin Company Inc. 2006) produced an estimate that natural attenuation processes active in groundwater in the area of Monitoring Well MW- 3, which is located a few feet down gradient from the former fueling station pump island (see Figures 1 and 2) where the releases of fuel hydrocarbons occurred, will fully remediate the BTEX compounds by 2014 and remove all components of diesel by 2008 and all components of gasoline by 2022. SIC has previously cautioned that those estimates are fragile, based as they are on numerous simplifying assumptions about the effects of very complex natural processes that are highly sensitive to temporal site- specific geochemical and microbiological conditions of the subsurface. However, they do indicate that in areas of the site where aerobic conditions in groundwater are sustained, natural attenuation will remediate the subsurface within a reasonable period; therefore it is an optimal choice for remediation of the contaminated subsurface at 7500 West Eleventh Street. 2.3.2 Solubility of Diesel and Gasoline in Groundwater It is instructive to consider the measured concentrations of diesel and gasoline in groundwater in Monitoring Well MW-7 and in other areas of the site. Both diesel and gasoline are, of course, mixtures of over 200 individual hydrocarbon compounds, each of which has a specific solubility in groundwater. Those solubilities, although all are very low, can vary significantly depending upon the 'physical and chemical condition of the groundwater (e.g., temperature, pH, dissolved solids), the physio-chemical properties of the soil and, most importantly, of the specific components and the fraction of those components that are present in the fuel product in its in situ condition in the subsurface. Those conditions can also vary dynamically in the subsurface as seasons change and the concentrations of components of the fuels are modified with time by processes colloquially known as "weathering." For the reasons stated above, it is difficult to predict the solubility of a specific fuel or mixture of fuels in groundwater beneath a specific site. However, a.range of estimates can be offered. The solubility of a fuel hydrocarbon is usually stated as the mean