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y Stantec ,,, SECOND QUARTER 2008 GROUNDWATER MONITORING REPORT Background and Previous Work L July 7,2008 � 2.0 Background and Previous Work The Site has been used by PG&E as a general construction yard for pipeline crews since the late 1950s and was purchased by PG&E in 1991. In June 2005, Stantec performed a Phase I Environmental Site Assessment (Phase I ESA) at the Site (SECOR, 2005). The Phase I ESA determined that in 1995, PG&E removed one concrete AST from the Site. The AST was a ConVault-style tank consisting of separate vessels for storage of diesel and gasoline. According to on-site PG&E employees, the ConVault had reportedly replaced older steel ASTs used for storing and dispensing fuels. bw The following sections summarize subsurface investigations performed to date by Stantec. 2.1 DECEMBER 2005 INVESTIGATION L In December 2005, Stantec completed a limited subsurface investigation to evaluate soils beneath the former ConVault and steel ASTs for the presence of fuel hydrocarbons. The locations of the former ConVault AST and soil sample locations are illustrated on Figure 2. Minor concentrations of diesel- and motor oil-range petroleum hydrocarbons were detected in shallow soils to 2 feet below ground surface (bgs), and soluble fractions of these constituents were not present at or above the method reporting limits. Shallow soils consisted of silts and clays, underlain by a 'hardpan' layer at approximately 3.5 feet bgs. Based on the low chemical concentrations, lack of reportable soluble fractions, and fine-grained soil types, Stantec concluded that the reported impacts had a very low potential to migrate to groundwater. The findings of the investigation were presented in the Stantec document entitled, "Results of Limited Phase 11 Investigation,"dated February 14, 2006 (SECOR, 2006a). 2.2 MAY 2006 INVESTIGATION Following the December 2005 investigation, additional information was obtained by PG&E and Stantec indicating that the older steel ASTs were not located at the ConVault location that was the subject of the 2005 investigation. The older ASTs were reportedly located in a nearby structure identified historically as the `heavy equipment storage shed.' In May 2006, Stantec v completed an additional subsurface investigation in the vicinity of the storage shed. Soil sample locations are illustrated on Figure 2. Using hand tools, seven soil borings were advanced to depths of approximately 3.3 to 3.8 feet bgs, where each of the soil borings encountered the �• hardpan layer. Thirteen soil samples were collected from representative depths above the hardpan and submitted for chemical analysis. The findings of the investigation were presented in the Stantec document entitled, "Results of Second Limited Phase II Investigation,"dated �+ August 2, 2006 (SECOR, 2006b). r is iApg&e\french camp\groundwater monitoring\may 2008 monitoring\gmr rpt_fnl.doc 2.1 4