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ENVIRONMENTAL HEALTH ]DEPARTMENT SAN JOAQUIN COUNTY Unit Supervisors Donna K. Heran, R.E.H.S. N: Director 304 East Weber Avenue,Third Floor Carl Borgman,R.E.H.S. Al Olsen, R.E.H.S. Stockton,California 95202-2708 Mike Huggins, R.E.H.S., R.D.I. • ., -a'P • Program Manager Telephone: (209)468-3420 Douglas W.Wilson, R.E.H.S. 4LjF�'R Laurie A. Cotulla, R.E.H.S. Margaret Lagorio,R.E.H.S. Program Manager Fax: (209) 464-0138 Robert McClellon, R.E.H.S. Website:www.sjgov.org/ehd/ Jeff Carruesco, R.E.H.S. DARIN ROUSE AUG 2 9 2005 CHEVRON TEXACO ENVIRONMENTAL MANAGEMENT COMPANY P O BOX 6004 SAN RAMON CA 94583-0804 RE: Chevron Service Station 9-1452 334 East Main Street Ripon, California San Joaquin County Environmental Health Department (EHD) has reviewed Work Plan Addendum for Additional Assessment, dated July 28, 2005, prepared by Secor International Incorporated (Secor) on your behalf for the above-referenced site. In this work plan, Secor proposes to install two monitoring wells (MW-9 and MW-10) to further evaluate the lateral extent of dissolved petroleum hydrocarbons east and west of the site. EHD approves the installation of MW-9 located adjacent to this site at 338 East Main Street. The total well depth will be approximately 40 feet below ground surface (bgs) and the well will be screened from approximately 25 to 40 feet bgs. EHD does not approve the installation of MW-10 at the proposed site at 212 Main Street, Ripon, California, in excess of 800 feet from the subject site. EHD believes this well should be relocated closer to the Chevron site and recommends MW-10 be installed approximately 400 feet west of the site at what appears to be a dirt road paralleling the south on-ramp to Highway 99. EHD believes that the originally proposed location for MW-10 is too distant for the data to have a clear context with the subject site. If MW-10 cannot be relocated closer to the site because of accessibility and safely issues, then perhaps the Nestle wells will help provide the hydrogeological context for the currently proposed well location. By e-mail dated July 20, 2005, Secor proposed discontinuing the weekly batch extractions at the above-referenced site. Secor stated that methyl tertiary-butyl ether (MTBE) has declined and stabilized at low concentrations in groundwater samples collected from extraction wells MW-6S and MW-8. EHD has reviewed the batch extraction data collected monthly from November 2003 through April 2004, and weekly from August 2004 through May 2005 and concludes that contaminant concentrations in extracted groundwater are still fairly high, and the current remedial action provides the only control on a plume of unknown mass and lateral extent. Therefore, EHD will not approve a reduction of extraction frequency until it can be shown that weekly extractions are no longer effective for managing the plume of dissolved MTBE. Chemicals of concern have not been detected in MW-1, MW-2, MW-3, MW-51), and MW-71)during any of the sampling events reported from 2003 to the present. Therefore, EHD will consider a reduction of the sampling frequency if proposed and justified for each well by your consultant.