SITE INFORMATION AND CORRESPONDENCE FILE 2

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,-..w, �..� San Joaquin County D Environmental Health Department DIRECTOR 1� Donna Heran,REHS r. --� 600 East Main Street Stockton, California 95202-3029 PR GRAMcCIOORDREHSORS I ... — �: Jeff Carruesco,REHS,RDI cq i F o �P Website:www.sjgov.org/ehd Kasey Foley,REHS Phone: (209)468-3420 Fax: (209)464-0138 M May 28 2010 u ! I JUS! 0 2 2010 1 Mr. Sergio Morescalchi Atlantic Richfield Company By I PO Box 1257 San Ramon, CA 94583 Subject: 76 (Former BP) Service Station#11192 LOP Case#:1995123 1403 Country Club Blvd Global ID#: T0607700329 j Stockton CA, 95204 U #: 2413 The San Joaquin County Environmental Health Department (EHD) has eviewed Revised Work Plan Addendum for Additional Site Assessment (Plan), dated April 6, 2010, and has the following comments. The Plan was submitted as required by the EHD since.the originally approved work plan, dated October 2005, and Work Plan Addendum for Additional Site Assessment (Addendum) dated March 26, 2009, had not been enacted. The 2005 plan called for installing six additional monitoring wells on and offsite at various depths to delineate contaminated ground water and was approved by the EHD in correspondence dated November 9, 2005. The work was never started; the 2009 Addendum proposed a reduction of the approved scope of work to the installation of just one B-zone monitoring well, MW-14, approximately 350 feet away from the site, screened from approximately 35' to 45' bgs (feet below ground surface). The EHD commented on the Addendum by letter dated July 9, 2009, and required a proposal to delineate the extent of the contamination in the groundwater. Tert-butyl alcohol (TBA) is the main constituent of concern at this site and is mostly impacting the C-zone (68.5' to 78.5' bgs) at the northeastern edge of the site. Elevated concentrations of TBA (the most current data from February 2010 indicates 11,000 micrograms per liter) have been present in the groundwater samples collected from MW-11, the only monitoring well onsite screened in the C-zone. Since August 2003, the groundwater samples from MW-11 have consistently contained elevated concentrations of TBA and the EHD has required the lateral and vertical extent to be delineated. Historically, the groundwater flow direction at this site has been northeasterly and the TBA plume appears to have migrated offsite towards Pershing Ave, and beyond, as indicated by the Shell site's monitoring wells (S-5 and S-15) located on Elmwood Avenue, east of Pershing Avenue. In correspondence dated July 9, 2009, the EHD expressed concern with the lack of lateral and vertical groundwater plume delineation around MW-11, determined that the Addendum was not adequate, and required 76/BP to submit a proposal that addressed all of the EHD concerns.