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CAP Addendum:Former Fue Station, 7500 West Eleventh Street, Tracy, C)1-W Page I 1.0 PURPOSE This document is an addendum to the Corrective Action Plan for the Oak Walk Redevelopment Site (The San Joaquin Company Inc. 2007). It has been prepared in response to a directive issued in a letter dated August 2, 2007 by the San Joaquin County Environmental Health Department (SJCEHD) (San Joaquin County Environmental Health Department 2007). This addendum addresses technical comments in the SJCEHD letter regarding the Corrective Action Plan. A copy of the SJCEHD letter is included as Appendix A of this document. 2.0 RESPONSE TO TECHNICAL COMMENTS Technical comments for which responses were requested by the SJCEHD in its letter are addressed below. 2.1 Recoverable Volume of LNAPL In December 2003, SJC estimated the extractable volume of light non-aqueous phase liquid (LNAPL) present in the subsurface beneath the 7500 West Eleventh Street Site due to the discharge of gasoline and diesel fuels to be on the order of 1,000 - 8000 gals. Since that estimate was made, additional groundwater-quality monitoring wells have been installed and the groundwater-quality monitoring program being conducted at the site has generated additional data regarding the distribution and thickness of LNAPL. 2.1.1 Apparent Thickness of LNAPL Table 1 shows the thickness of floating product (LNAPL) measured in eight wells, the locations of which are shown on Figure 1. No floating product has been detected in any well other than Monitoring Well MW-7. In that well, the apparent thickness of floating product over the period of April 2002 through November 2003 ranged from 0.13 to 0.58 ft. However, those apparent thicknesses have reduced following the periodic purging of LNAPL from that well that was initiated in November 2003. Over the 16-month period from March 29, 2006 through July 30, 2007, which period included the last four semi- 'r annual measurements of floating product thickness, the maximum apparent thickness measured was 0.05 ft. That value will be used in making a new estimate in the total extractable volume of LNAPL in the subsurface. 2.1.2 Thickness of LNAPL in Formation As was described in the Corrective Action Plan (CAP), the thickness of floating product measured in a well is greater than the actual thickness of LNAPL in the surrounding formation. Numerous methods are available to estimate the actual thickness of floating product in the formation compared to the thickness measured in the monitoring well. However, all of the methods have serious limitations and frequently provide highly inaccurate results. In addition, they frequently break down when small measured thicknesses of LNAPL are.considered. For example, the method developed by Hall, et al