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PG&E General Construction Ya.� -rench Camp - 2 - 28 February 2011
<br /> area contained the maximum soil concentrations of TPHd, TPHmo, and TPH as gasoline
<br /> (TPHg) at 24,000 milligrams per kilogram (mg/kg), 15,000 mg/kg, and 1,600 mg/kg,
<br /> respectively. PG&E observed visual evidence of contamination to the maximum depth
<br /> explored )f 3.5 feet bgs, where it encountered a hardpan layer. Figure 2 shows the locations
<br /> of the soi borings.
<br /> In February 2007, PG&E excavated about 40 cubic yards of soil in the area south of the heavy
<br /> equipment storage area where the maximum soil concentrations were previously detected.
<br /> Figure 2 shows the location of the excavation. During the excavation, PG&E determined that
<br /> the petroleum hydrocarbon contaminated soil was more laterally extensive that originally
<br /> anticipated and extended to depths greater than eight feet bgs. As shown in Table 1, the soil
<br /> sample collected at eight feet bgs contained TPHd, TPHmo, and TPHg at concentrations of
<br /> 7,800 mg/kg, 2,900 mg/kg, and 340 mg/kg, respectively.
<br /> In April 2007, PG&E completed a multiphase investigation that included a geophysical survey,
<br /> limited subsurface investigation, and installation of three monitoring wells. PG&E conducted
<br /> the geophysical survey to attempt to identify any remaining underground storage tanks (USTs)
<br /> or associated piping onsite. No subsurface anomalies consistent with USTs or related fuel
<br /> distributic n equipment were detected. PG&E also drilled five soil borings within and adjacent
<br /> to the February 2007 excavation area to characterize the extent of the petroleum hydrocarbon
<br /> contamination. As shown in Table 1 , PG&E detected TPHd, TPHmo, and TPHg in three of the
<br /> borings at about 20 feet bgs at maximum concentrations of 620 mg/kg, 6,500 mg/kg, and
<br /> 2,000 mg/kg, respectively.
<br /> In July 2C109, PG&E drilled four soil borings to 20 feet bgs and collected soil samples to
<br /> confirm the potential limits of the proposed excavation to remove soil contamination above the
<br /> California Regional Water Quality Control Board — San Francisco Bay Region Environmental
<br /> Screening Levels (ESLs) for gross contamination. Figure 2 shows the soil boring locations.
<br /> As shown in Table 1 , PG&E detected TPHd, TPHmo, and TPHg at maximum concentrations
<br /> of 14,OOC mg/kg, 22,000 mg/kg, and 2,800 mg/kg, respectively. Soil concentrations from
<br /> SB-5, adjacent to the property boundary, indicated that concentrations may have migrated
<br /> onto the adjacent residential property. .
<br /> In October 2009, PG&E drilled five soil borings to 20 feet bgs on the property adjacent to the
<br /> Site to dE,termine the extent of soil contamination. The 14-foot bgs sample from boring SB-14
<br /> contained the maximum concentrations of TPHd and naphthalene at 202 mg/kg and
<br /> 5.7 mg/kcj, respectively. These concentrations are below the gross contamination ESI-s. The
<br /> 20-foot bgs sample from the same boring did not contain petroleum hydrocarbons above
<br /> laborator/ detection limits. Table 2 presents the October 2009 soil data.
<br /> In March 2010, PG&E drilled three soil borings adjacent to the locations of previous borings
<br /> with high petroleum hydrocarbon concentrations in soil. Each boring was advanced to
<br /> groundwater, and soil samples were collected and analyzed for petroleum hydrocarbons. As
<br /> shown in Table 1 , the maximum concentrations of TPHd were detected in the soil samples
<br /> collected from 11 feet bgs. Soil concentrations decreased to below laboratory detection limits
<br /> in the 26-foot bgs and deeper samples. Depth to water is about 28 feet bgs.
<br /> In September 2010, PG&E excavated about 200 tons of contaminated soil to a depth of
<br /> 11 feet bgs and backfilled the excavation with clean fill. PG&E estimates that about
<br /> 86 tierce it of the petroleum hydrocarbon mass was removed and that soil concentrations
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