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,.,. Work Plan for Refined Plume Definition and Management of Floating Product-7500 W 11th St., Tracy, CA. Page 13 that removal of floating product using the technology proposed by SJC would not meet with that Agency's approval because "it has not been done before in San Joaquin County" (Lagorio 2002). Because we know it to be based on well-founded hydrogeological and geotechnical engineering principals and has been shown to be highly effective in practice, including at the L, Navarra Site when floating product was removed from the subsurface beneath the south side of West Eleventh Street (see Section 2.3), SJC included its technical proposals to remove floating product from the subsurface on the north side of West Eleventh Street in its formal report of the extended site characterization program (The San Joaquin Company 2002c). That recommendation, including further explanation of the engineering techniques that would be applied, was repeated in the quarterly report submitted to the SJCEHD for the groundwater- i.- quality monitoring round conducted on January 20, 2003 (The San Joaquin Company 2003c). That quarterly monitoring report also included a request that SJCEHD issue written direction for design and implementation of a floating product removal program so that it could be a— submitted by the Navarras to the USTCF in order to obtain funding for that work. On April 22, 2003, SJCEHD issued the requested written directive for preparation of a formal work pian for extensions of the environmental management program at the 7500 West Eleventh Street property (San Joaquin County Environmental Health Department 2003). This present document has been prepared in response to that directive. However, the letter of direction, inter alfa, specifically disapproved of the technical approach for removal of floating product from the subsurface on the north side of West Eleventh Street that had been previously recommended by SJC, and had been successful during the tank and piping ""io' removal process (see Section 2.3). 5.2 Estimates of Thickness and Volume of LNAPL in Subsurface Numerous techniques have been developed to make estimates of the volume of product floating on a plume of groundwater that is affected by petroleum hydrocarbons. However, ` none apply to all soil types and all plume types and produce notoriously inaccurate results (United States Environmental Protection Agency 1996). Nevertheless, due to the lack of more robust methods, they are frequently used in practice to make order of magnitude assessments of the total volume of floating product that must be managed at a given site, which is important when considering the applicability of different technologies for remediation of LNAPL. Although the calculations involved are quantitative in nature, the process of estimation is, in fact, heavily influenced by qualitative factors related to field observation and a synthetic evaluation of a site's hydrogeologic regime that can only be_ made by an experienced engineer. The following Sections describe some of the quantitative and semi-quantitative calculations used by SJC as guides to estimate the volume of floating product currently present on the water table in the area around the north side of West Eleventh Street. SJC