SITE INFORMATION AND CORRESPONDENCE

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PG&E Ger eral Construction Ya,`.,, French Camp -4 - 28 February 2011 SOIL VAPOR INVESTIGATION ACTIVITIES In September 2009, PG&E installed three dual-nested soil vapor wells and collected soil vapor samples at five and 12 feet bgs. Several soil vapor samples exceeded the ESLs for naphthalene. The maximum concentration of naphthalene was 990 micrograms per cubic meter (pcl/m). PG&E conducted a risk assessment to evaluate the vapor intrusion pathway using the maximum detected soil gas concentrations and reference doses approved by the Departmont of Toxic Substances Control for specific compounds. Based on that risk assessment, PG&E concluded that there is no unacceptable risk from vapor intrusion to current o-future receptors. In addition, in 2010, these soil gas sampling locations were excavated to 11 feet bgs. Figure 2 shows the location of the soil vapor wells, and Table 5 presents the soil gas sample results. In November 2009, PG&E drilled three dual-nested soil gas probes on the adjacent property. Petroleum hydrocarbons in soil vapor did not exceed ESLs and do not pose a threat to human health or the environment. Table 6 presents the results of the soil gas samples collected on the adjacent property. SUMMARY The Site s a former general construction yard owned by PG&E. PG&E continues to use the Site for s,orage or as a staging area for regional operations, but the Site does not support day to day operations. Between 2006 and 2010, PG&E drilled 27 soil borings at the Site and the adjacent property. Soil samples contained TPHd, TPHmo, and TPHg at maximum concentrations of 25,000 mg/kg, 22,000 mg/kg, and 2,800 mg/kg, respectively. These concentrations were between night and 11 feet bgs. In 2010, PG&E excavated about 200 tons of contaminated soil to a c epth of 11 feet bgs and backfilled with clean fill. PG&E estimates it removed about 86 percent of the petroleum hydrocarbon mass at the Site and that the remaining mass does not pose a threat to human health or the environment. Direct contact with the contaminated soil is not a complete exposure pathway, and modeling indicates that remaining concentrations will not migrate to groundwater or cause vapor intrusion concerns. In May 2007, PG&E installed three groundwater monitoring wells and sampled them quarterly until August 2008. MW-1 contained the maximum concentration of TPHd and TPHg in May 2007 at 850 pg/L and 580 pg/L, respectively. Since May 2007, only the February 2008 sample it MW-1 was above the TPHd water quality objective (WQO) of 100 pg/L. In August 2008, all Site wells were below WQOs, and in July 2009, PG&E properly abandoned the monitoring wells. In 2009, 13G&E installed six soil vapor wells and collected soil vapor samples from the Site and adjacent aroperty. Several soil samples exceeded the ESLs for naphthalene, and PG&E conducted a risk assessment to evaluate the potential for indoor air exposure from contaminated soil and groundwater. PG&E concluded that remaining soil and groundwater concentrations do not pose a threat to human health from soil vapor. PG&E removed the ALTs and excavated contaminated soil. Remaining soil and soil gas concentrations do not pose a threat to human health or the environment. Site groundwater monitoring well concentrations were below WQOs when they were destroyed in 2009. Central Valley Water Board staff concurs that no further action is needed at this Site.