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*« Work Plan far Refined Plume Definition and Management of Floating Product-7500 W 11th St., Tracy, CA, Page 6 4.0 SITE CHARACTERIZATION AND GROUNDWATER-QUALITY MONITORING �. Characterization of affected zones of the subsurface at the 7500 West Eleventh Street site has advanced in stages, beginning with observations made at the time that the underground tanks were removed in 1998. 4.1 Initial Site Characterization Following discovery of the unauthorized release of fuel hydrocarbons at the Navarra Site in December 1998, the Navarras retained SJC in February 1999 to provide engineering services for site characterization, site remediation and compliance with California regulations. i. In early 1999, SJC contacted the SJCEHD and outlined, in informal summary form (The San Joaquin Company 1999b), a recommended technical approach for site characterization and prompt remediation of the 7500 West Eleventh Street site based on a risk-based corrective action (RBCA) analysis (American Society for Testing and Materials 1998). SJC's early communication with the SJCEHD regarding the proposed technical approach was made according to our long-standing standard practice for environmental management of such sites, which, shortly prior to SJC's first contact with SJCEHD, had also been adopted by the United States Environmental Protection Agency (USEPA) in its guidance documents for expedited site assessments for underground storage tank sites (United States Environmental Protection Agency 1997). However, in April 1999, SJCEHD responded to SJC's strategic proposal by noting that the RWQCB recognizes all waters as potentially beneficial, that California UST regulations require cleanup goals that will protect such uses and that a RSCA-based approach could not be taken until additional site characterization work had been undertaken to define the extent of contamination at the site (San Joaquin County Environmental Health Department 1999b). In the same guidance document, SJCEHD directed that a formal work plan structured around a conventional procedural, rather than site-specific engineering, approach be prepared. In response to SJCEHD's request, SJC then developed a comprehensive work plan to characterize the Navarra Site and delineate the extent of the plume of affected groundwater (The San Joaquin Company Inc. 1999a). That work plan, submitted in early June 1999, provided for drilling of some 20 small-diameter exploratory borings and the installation of 8 groundwater-quality monitoring wells that included wells and small-diameter exploratory borings to be located on the east side of Chrisman Road, south of West Eleventh Street, and on both sides of Chrisman Road where it runs north from West Eleventh Street. The locations of the proposed exploratory borings and monitoring wells were specifically selected by the California-registered Geotechnical Engineer in responsible charge of the work to be distributed such as to delineate adequately the lateral limits of the plume of affected groundwater, which, based on elementary principals of hydrogeology and consideration of the local and regional groundwater flow regime, could be anticipated to have migrated to the east of the western side of Chrisman Road and north across West Eleventh Street. In late July 1999, SJCEHD reviewed SJC's site characterization work plan and, with minor AMP sic