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® AVIN NEWSOM <br /> Pâť‘VrrtNO,t <br /> 070 <br /> Water Boards ENV E GAR CIA <br /> ENVIRONMENTAL PROTECTION <br /> Central Valley Regional Water Quality Control Board <br /> 25 September 2023 <br /> Casey Sanders <br /> AECOM <br /> 300 Lakeside Drive, Suite 400 <br /> Oakland, CA 94612 <br /> REVIEW OF VAPOR INTRUSION ASSESSMENT WORK PLAN, BP/APPLIED <br /> AEROSPACE STRUCTURES CORPORATION, 3457 SOUTH AIRPORT WAY, <br /> STOCKTON, SAN JOAQUIN COUNTY <br /> Central Valley Regional Water Quality Control Board (Central Valley Water Board) staff <br /> have reviewed the 18 August 2023 Vapor Intrusion Assessment Work Plan (Work Plan) <br /> for the BP/Applied Aerospace cleanup case located at 3457 South Airport Way in <br /> Stockton (Site). <br /> Background <br /> Aerospace manufacturing and application previously occurred at the Site, and historical <br /> documents indicate known groundwater and soil contaminations of volatile organic <br /> compounds (VOCs) such as 1 ,1-dichloroethylene (1,1-DCE), 1 ,1-dichloroethane (1,1- <br /> DCA), 1,1,1-trichloroethane (1,1,1-TCA), 1,1,2-trichloroethane (1,1,2-TCA), chloroform, <br /> tetrachloroethylene (PCE), and trichloroethylene (TICE). As a result, in a letter dated 8 <br /> March 2023, the Central Valley Water Board requested the addition of 1,4-dioxane <br /> (dioxane) in the next groundwater sampling and the inclusion of a vapor intrusion <br /> assessment in efforts to move the Site closer to no further action. <br /> Sub-Slab Soil Gas Investigation <br /> The Work Plan proposes installing 12 permanent sub-slab gas probes, with two probes <br /> each in Buildings 6 and 9, one each in Buildings 2 and 3, and six in Building 4. Probes <br /> that are located immediately below the slab will be installed by removing existing floor <br /> covering and drilling a 1-inch-diameter hole through the building floor slab and will <br /> advance 3 to 4 inches to accommodate the probe installation. In addition to gas tight <br /> fitting and flush-mounted boxes, sampling probes will be created using a 1/8- or 1/4-inch <br /> diameter tubing with a permeable probe tip at the end. Sand packs will fill and center <br /> the probe tip, leaving at least 6 inches of sand above and below the level of the probe <br /> tip. A dry granular bentonite transition seal will then be added on top of each sand pack. <br /> Cement mixed with bentonite will fill the remainder of the hole. <br /> MARK BRADFORD, CHAIR I PATRICK PULUPA, Es4., EXECUTIVE OFFICER <br /> 11020 Sun Center Drive#200, Rancho Cordova, CA 95670 1 www.waterboards.ca.gov/centralvalley <br />