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ARCADIS Ms. Kathy Amaru <br /> October 19, 2010 <br /> Summary and Discussion <br /> ARCADIS directed the additional removal of approximately 48 cy of soil impacted <br /> with petroleum hydrocarbons from the December 2009 excavation at the PG&E <br /> McDonald Island Compressor Station, C-51 Tank site. Soil was removed from depths <br /> ranging from 7 to 11.5 feet bgs. The excavation area was subsequently backfilled <br /> with clean fill and the surface was restored to surrounding conditions. One soil boring <br /> was advanced near the excavation and converted into a temporary well point for <br /> groundwater sampling. <br /> Nine soil samples were collected from the excavation sidewalls and bottoms. Final <br /> confirmation soil sampling results indicate that concentrations of TPH as gasoline <br /> and diesel were not detected above the established screening levels after all <br /> excavation activities had been completed.Two samples collected from the SB- <br /> 02/SB-13 excavation area contained concentrations of TPH as motor oil above the <br /> screening level. <br /> Confirmation soil sampling results indicate that the extent of soil that remains <br /> impacted with TPH as motor oil is confined to the limited area between the current <br /> SB-02/SB-13 excavation area and several subsurface utilities. This area is located <br /> adjacent to the north-northwest vicinity of the December 2009 excavation and is <br /> approximately 5 feet wide by 8.5 feet long. Additional excavation in this area is not <br /> possible due to the proximity the subsurface utilities which include active electrical, <br /> gas and water lines. Further soil removal in this area would likely undermine these <br /> adjacent structures and pose a significant safety risk to onsite personnel. This <br /> precludes further excavation near SB-02/SB-13. <br /> The results of the grab groundwater sample collected north of the excavation showed <br /> concentrations of TPH as diesel slightly above the RWQCB ESL. The results for TPH <br /> as gasoline, TPH as motor oil, and naphthalene were all below the RWQCB ESLs in <br /> the groundwater sample collected from the site. <br /> The McDonald Island Compressor Station is not scheduled for decommissioning and <br /> will remain operational for the foreseeable future. Therefore, conditional closure of <br /> the site is recommended, with the understanding that impacted source material will <br /> be removed if the substation is decommissioned, making safe removal of residual <br /> impacts possible. <br /> Page: <br /> 0111012775 as 10/12 <br />