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Mr. Jeff Whitworth - 2 - 8 April 2008 <br /> the 6 foot vapor samples at a concentration of 4.0 pg/m3 and was below the detection limit of <br /> 2.5 Ng/m3 in the remaining samples. Other constituents detected in the vapor samples <br /> included toluene, ethylbenzene, total xylenes, naphthalene, and Freon 134a (used as a leak <br /> detection compound) at various concentrations and depths. None of the 6-foot benzene <br /> concentrations were greater than the California Human Health Screening Levels (CHHSLs). <br /> Deeper and higher concentration soil vapor samples were further analyzed using the Johnson <br /> and Ettinger Model for Evaluating Subsurface Vapor Intrusion into Buildings. The highest <br /> benzene concentration (390,000 pg/m3) from B-72 at 30 feet resulted in a calculated <br /> incremental risk of 3.4 x 10-4 if a building were to be placed over this location. Lower <br /> incremental risks of 1.6 x 10-6 and 2.1 x 10-5 were calculated for the highest concentrations <br /> from nearby borings B-70 and B-71. Shell recommends future evaluation of the potential risk <br /> to indoor air when construction plans are available for the property. <br /> REGIONAL WATER BOARD REVIEW COMMENTS <br /> Regional Water Board staff notes that the extent of the groundwater plume is not defined as <br /> evidenced by the concentrations in B-75. Therefore, by 6 June 2008, submit a work plan to <br /> propose additional investigative activities to further define the extent of the plume. In addition, <br /> a sensitive receptor survey is required to identify and map water supply wells and, surface <br /> water bodies within one mile of the pipeline release. The sensitive receptor survey should <br /> include a records search of well completions reports with the Department of Water Resources. <br /> A field survey should also be conducted to observe and verify any water supply wells <br /> identified in the records search and to identify and locate any additional water suoply wells for <br /> which records are not available. Attempts should be made to obtain the depth of.the annular <br /> seal and the total depth of each of the wells identified. The results of the sensitive receptor <br /> survey are to be submitted by 7 July 2008. If you have any questions regarding this letter, you <br /> may contact me at (916) 464-4665 or ncasebeer@waterboards.ca.gov. <br /> Nathan Casebeer, P.G. - <br /> Engineering Geologist <br /> Private Sites Cleanup Unit <br /> cc: Mr. Michael Hurd, Scientific Applications International Corporation, Oakland <br /> Mr. Mike Infurna, San Joaquin County Environmental Health Department, Stockton <br /> Ms. Eileen Nottoli, Allen Matkins Leck Gamble & Mallory LLP, San Francisco <br /> Ms. Ileana Rhodes, Shell Global Solutions (US) Inc., Houston, TX <br /> Mr. David Corliss, Golden State Developers, Walnut Creek <br /> NECNd'.\slit dodrAustinSlaff Nathan\Shell-Hansen B Schulte Rd\8-6-07 Investigation Report.doc <br />