SITE INFORMATION AND CORRESPONDENCE FILE 2

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r ( I San Joaquin County DIRECTOR . P „ Environmental Health Department Donna Reran, REHS cp M ASSISTANT DIRECTOR a � 600 East Main Street Laurie Cotulla,REHS { Stockton, California 95202-3029 PROGRAM COORDINATORS • _ r• Mike Huggins,REHS, RDI Mararet rio,REHS Website:www.sjgov.org/ehd FOR Robert M c euon, REHS Phone: (209)468-3420 Jeff Carruesco,REHS, RDI Fax: (209)464-0138 Kasey Foley,REHS April 29, 2009 Ms. Stephanie Furgal Chevron Environmental Management Company 6111 Bollinger Canyon Road, Room 3652 San Ramon, CA 94583 Subject: Former Chevron Service Station 9-0557 139 South Center Street Stockton, California The San Joaquin County Environmental Health Department (EHD) has reviewed current and historical data submitted for the above-referenced site by ARCADIS on your behalf. Groundwater samples collected from the deepest well, MW-18D2, installed in November 2007 and screened between 100 and 105 feet below surface grade (bsg), and grab groundwater samples collected from boring CPT5-W-116 at 116 feet bsg and CPT6-W-110 at 110 feet bsg indicate that contaminants of concern have not been delineated vertically and laterally in groundwater at this site. Also, based on a cursory review of soil analytical data, it does not appear that soil is delineated vertically near the former underground storage tanks (USTs)removed in 1990. The EHD believes a feasibility study (FS) is needed to identify a remedial technology, or a combination of technologies, capable of mitigating petroleum hydrocarbon contamination in the area of the former USTs, specifically in the area of MW-15 and MW-15D. Submit the FS to the EHD by July 30, 2069, and include methods and calculations .for estimating the contaminant masses in groundwater and soil as part of this study. Before proceeding with additional site investigation, the EHD recommends submittal of a preliminary site conceptual model (SCM). The general purposes of an SCM are to demonstrate where the contaminants came from, where they.are at the present time, how they move through the subsurface, how they will respond to changes in the groundwater .flow characteristics or to potential remediation efforts, what the contaminants' ultimate environmental fate will be, and to help evaluate the risk posed by the contaminants to groundwater supplies and sensitive receptors. To be sure that the SCM will adequately address the assessment, remediation and, ultimately, final closure requirements for this site, the following should be included in the SCM: • Local and regional plan view maps showing locations of sources, boring and monitoring well locations, lines of cross section, extent of contaminants in each media (i.e., an interpretive drawing — not merely a plot of laboratory results), direction and rate of groundwater flow, and receptor locations, including water supply wells within 2,000 feet of the site;