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Ms. Anne Conner S%.W -2 - 16 December 2009 <br /> PG&E French Camp <br /> The Email includes a summary of the risk assessment. PG&E calculated a risk for the direct <br /> contact and vapor intrusion pathways using the Department of Toxic Substances Control's <br /> approved References Doses for specific compounds. The risk assessment concluded that <br /> there is no unacceptable risk under the existing conditions. PG&E then concluded that <br /> excavation to 10 feet bgs will eliminate any nuisance conditions and no site-specific cleanup <br /> goals are necessary. <br /> Our comments are included below: <br /> 1. In the 17 November 2009 conference call, Central Valley Water Board staff stated we <br /> were concerned about the increasing concentrations with depth in soil borings SB-5, <br /> SB-7, and SB-8. SB-5 and SB-7 are on the edges and SB-8 is near the center of the <br /> excavation area. The maximum concentrations in these borings are below 10 feet bgs <br /> and will not be removed during the excavation. Central Valley Water Board staff is <br /> concerned that PG&E has not vertically defined the soil contamination at the Site. By <br /> 1 February 2010, PG&E needs to prepare a work plan to define the vertical extent of <br /> soil contamination at the Site. <br /> 2. Central Valley Water Board staff is concerned that the TPH left in soil after excavation <br /> may leach to the groundwater. Depth to water is about 30 feet bgs. By 15 April 2010, <br /> PG&E needs to conduct modeling using the soil concentrations proposed to remain <br /> following excavation to determine if the TPH will migrate to groundwater. If the TPH will <br /> migrate to groundwater, PG&E needs to determine if the resulting groundwater <br /> concentrations will exceed water quality objectives. <br /> 3. PG&E conducted the risk assessment using the carbon chain fractionation and not the <br /> bulk TPH values. The carbon chain fractionation concentrations are much lower than <br /> the TPH concentrations. It is unclear why there is a disparity between the carbon chain <br /> data and the TPH data, but PG&E needs to conduct the risk assessment using the <br /> more conservative values. <br /> In summary, by 1 February 2010, PG&E needs to prepare a work plan to define the vertical <br /> extent of the soil contamination. In addition, by 15 April 2010, PG&E needs to conduct <br /> modeling to show whether the remaining soil concentrations will leach to the groundwater. If <br /> you have any questions regarding this letter, you may contact me at (916) 464-4819 or by <br /> email at ksheltonna waterboards.ca.gov. <br /> KRISTI SHELTON, P.E. <br /> Water Resources Control Engineer <br /> Aboveground Tanks Cleanup Unit <br /> CC' Mr. Michael Infurna, San Joaquin County Environmental Health Department, Stockton <br /> Mr. Neil Doran, Stantec, Rancho Cordova <br />