SITE INFORMATION AND CORRESPONDENCE

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r San Joaquin County DIRECTOR Ap U r N Donna Heran,REHS Environmental Health Department ASSISTANT DIRECTOR Q % 600 East Main Street Laurie Cotulla,REHS Stockton, California 95202-3029 PROGRAM COORDINATORS Carl Borgman,REHS Website: www.sjgov.org/ehd Mike Huggins,REHS,RDI Margaret Lagorio,REHS Phone: (209)468-3420 Robert M Cellon,REHS Fax: (209)464-0138 Jeff Carruesco, REHS,RDI Kasey Foley, REHS October 22, 2008 Stephanie Furgal Chevron Environmental Management Company 6111 Bollinger Canyon Road Room 3652 San Ramon, CA 94583 Subject: Former Unocal #2859 1665 Pacific Avenue Stockton, CA The San Joaquin County Environmental Health Department (EHD) has reviewed the "Work Plan for Well Destruction and Soil Vapor Intrusion Investigation" (Work Plan) dated August 29, 2008, prepared by ARCADIS for the above subject site and provides the following comments. The Work Pian proposes installation of nine pairs of soil vapor probes (one shallow, one deep) based on a grid of nine zones. The shallow probes are to be installed at approximately six feet below ground surface (bgs) and at least one soil sample will be collected from each boring. The deep probes are to be installed at approximately twenty feet bgs and soil samples are to be collected at changes in soil stratigraphy. There is no technical justification provided for the proposed vapor probe locations other than the site was divided into grids and one pair of vapor probes would be installed in each grid, nor was technical justification provided for collection and analysis of soil samples at six feet in the shallow probes and changes of soil stratigraphy in the deep probes. In a letter dated June 18, 2008, the EHD directed that the mass of contaminants remaining in the soil be calculated and that it be shown that the remaining contamination is not a continuing source of contamination to the groundwater and that potential health hazards of direct exposure, vapor emission to indoor air and gross contamination be addressed. The placement of soil vapor probes should be near buildings and in areas of known or suspected contamination (former underground storage tanks, piping and dispenser locations). Table 1 in the "Conceptual Site Model" dated April 25, 2006, prepared by ENSR lists all the historical soil chemical analytical results and should be used to justify vapor probe locations. As previously stated by the EHD, high concentrations of contaminants (from depths of 11 feet to 51 feet) were documented in the soil in the former dispenser area when vapor well VW2 was drilled so one of the probes should be placed near VW2 which is near MW10 where the highest groundwater contamination is detected. Limiting the assessment of soil contamination to 20 feet bgs, when it has been documented that soil contamination existed from 11 to 51 feet in this area will not provide current information on the contaminant mass remaining in the soil at this site or address issues of gross contamination.