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CAP Addendum:Former Fuellf'lg Station, 7500 West Eleventh Street, Tracy, G Page 14 A concentration of 13,000 �ig/L of TPHd and TPHg is 74% of the concentration of fuel hydrocarbons detected in the groundwater in Monitoring Well MW-7 in July of 2007. This would suggest that what is needed is a 26% reduction in dissolved components of fuel hydrocarbons in the groundwater. More realistically, the reduction required, based on the concentration of fuel hydrocarbons in groundwater in Monitoring Well MW-7 in March 2006, which was 35,000 µg/L should be considered. The latter value was the greater due to the low groundwater elevation conditions that occur at the Site during the early months of the year when the thickness of LNAPL floating on the water table is usually at its greatest. Using the 35,000 µg/L value, the reduction in the total concentration of fuel hydrocarbons required to initiate aerobic processes of natural _ attenuation is estimated to be 63%. The question now becomes: "How can the requisite reduction in the concentrations of TPHd and TPHg in groundwater be achieved?" 2.3.4 Importance of Removal of LNAPL It is important to understand that to answer the question posed above, it is not very useful to ask the simplified question; "What is the mass of contaminants that must be removed?" This is because the mass of contaminants that is adsorbed onto the soil and that is in the form of LNAPL affect the groundwater in radically different ways. Consider the estimated total mass of 4,625,025 grams of contaminants in the subsurface over the area of the Navarra Site that is estimated to be affected by LNAPL that was developed in Section 2.2.1. Of that total, the mass of contaminants dissolved in groundwater was estimated to be 54 grams or 0.0012% of the total. The corresponding values for adsorbed contaminants are 2,105,265 grams or 45.52 % and contaminants in the form of LNAPL are 3,179,746 grams or 68.75%. As was theoretically estimated in Section 2.2.1.5.4, pumping 10,000 gallons from the extraction trench would remove 1,589,873 grains of contaminants in the form of LNAPL from the subsurface, 138,324 grams of contaminants would be removed by excavating soil from the extraction trench and 1.81 grams of dissolved contaminants would be removed in the pumped groundwater, for a total extraction of 1,728,199 grams. That amounts to 54.35% of the contaminant mass currently estimated to be present in the subsurface in the area of the site that is affected by LNAPL. It might naively be concluded that a contaminant mass reduction of 54.35%would induce aerobic natural attenuation in the groundwater in the affected area because it is well in excess of the required 26% reduction in the dissolved concentration of TPHd and TPHg estimated in Section 2.3.3 to be required based on the condition in the groundwater in Monitoring Well MW-7 in July of 2007. Although, it would not totally achieve the full percentage reduction sought, the same naive assumption might lead us to believe that removal of the same contaminant mass would go a very long way towards generating the required condition of a 63% reduction in the concentration of components of fuel hydrocarbons in groundwater in that well if the condition to be considered were to be that