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CAP Addendum:Former Fuellfg Station, 7500 West Eleventh Street, Tracy, Page 16 For the LNAPL component of the total contaminant mass in the subsurface and its contribution to dissolved contaminants in groundwater the situation is quite different. As long as LNAPL is present and in contact with groundwater, it will maintain the concentration of contaminants in the groundwater at the top of a vertical zone that extends some 10 ft. below the "smear" zone at concentrations close to the fuel's solubility limit (Chiang et. al. 1989, Robbins and Martin-Hayden 1991). Within that zone, contaminant concentrations will be at their greatest at its top and will decline with depth from there in a Gaussian distribution to the bottom of the 10 ft. deep zone (American Society for Testing and Materials 2004). As it happens, at the Navarra Site the distance from the bottom of the smear zone to the bottom of the contaminated aquifer is, on average, approximately 10 ft. (The San Joaquin Company Inc. 2006). As was cited in Section 2.3.2, depending upon the in situ conditions in the subsurface, the solubility of diesel fuel ranges from 1,000 to 3,000 µg/L and, depending upon the fraction of fuel oxygenates that it contains, the solubility of gasoline can range from 10,000 gg/L to 200,000 µg/L. Excluding results from analyses on samples of groundwater recovered from Monitoring Well MW-7 that SJC believes were affected by fine particles of emulsified LNAPL (which samples are often characterized by a distinctive gray coloration of the quiescent water), the concentrations of dissolved diesel fuel in groundwater in the area around that well have ranged up to 3,600 µg/L and the gasoline concentrations have ranged up to 37,000 µg/L. The causation of these high contaminant concentrations is obviously dominated by components of fuel hydrocarbons passing into solution from the LNAPL that is floating on the groundwater From the foregoing discussions, it is now clear that an appropriate measure of the performance requirements for the remediation technology proposed for the Navarra Site is not an estimate of the total mass of contaminants that can be removed from the subsurface but the degree to which LNAPL can be eliminated. In Section 2.2.1.1 a very conservative-estimate was made that 1,050 gallons of LNAPL may remain in the subsurface beneath the Navarra Site. In Section 2.1.6, the severe limitations of the state of the art for estimating the portion of total LNAPL affecting a specific site that can be recovered were explained and an order of magnitude estimate of 525 to 1,050 gallons for the volume of LNAPL that may recoverable from beneath the Navarra Site was established. If the actual recovery is some 525 gallons, that will represent half of the total LNAPL estimated to be present, while a recovery of 1,050 gallons would eliminate all of the LNAPL estimated to be present in the subsurface. If 50% of the LNAPL is recovered by pumping from the extraction trench, a transition from anaerobic to aerobic conditions in the groundwater in the vicinity of Monitoring Well MW-7 would not be achieved immediately. However, as was demonstrated by the evaluation of natural attenuation processes active at the Site, natural attenuation processes are active in that area although the processes there are anaerobic and are thus acting more slowly than in areas of the site where aerobic conditions prevail in the groundwater. A 50% reduction in the volume of LNAPL would halve the time required for aerobic conditions to be initiated, which is always accompanied by a rapid increase in the natural