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I T 77. Olt MAY 17 P ;f 2: Q May 13, 2004 Ms. Lori Duncan 00548 San Joaquin County Environmental Health Department 04c 304 East Weber Avenue, Third Floor Stockton, California 95202 Re: Modification of Remediation Activities ARCO Service Station No. 0545 Located at 1612 Hammer Lane, Stockton, California Dear Ms. Duncan : URS Corporation Americas (URS) is modifying the remediation activities for the Atlantic Richfield Company (ARCO — a BP affiliated company) Service Station No. 0548, located at 1612 Hammer Lane in Stockton, California. A soil vapor extraction (SVE) system began operation at this site in September 2000. Since it began operating, the system has removed over 2,000 pounds of total petroleum hydrocarbons as gasoline (TPHg). During first quarter 2004, three site visits were conducted to optimize and maintain the SVE/AS system. Influent TPHg concentrations have decreased from 310 parts per million by volume (ppmv) in November 2003 to 3 ppmv in February 2004. The most recent data for March 2004 was below detection limits for all constituents analyzed (Table 1). Groundwater contaminant concentrations have also decreased significantly. For instance, sampling in MW-5 during August 1996 identified 9,600 micrograms per liter (µg/L) TPHg, 510 µg/L benzene, and 39 µg/L methyl tert-butyl ether (MtBE). Since June 2002, all analytes in MW-5 have been below detection limits. In VW-1 , VW-2, and VW-3, TPHg was identified during September 1998 at 18,800 µg/L; 13,200 µg/L, and 39,300 µg/L, respectively. During the March 2004 sampling event, TPHg was not detected, 290 µg/L, and 1 ,000 µg/L, respectively. Table 2 presents the quarterly groundwater sampling analytical results and Figure 1 provides the most recent sampling data for the site. Based on both the declining SVE system influent concentrations and the groundwater contaminant concentrations, ARCO would like to discontinue the operation and monthly sampling of the remediation system. URS proposes that the SVE system be turned off but left in place. Quarterly groundwater sampling at the site will continue and the groundwater contaminant concentrations evaluated to determine whether there is a need to restart the system. In the event that contaminant concentrations increase significantly subsequent to the system being switched off, further remediation utilizing the SVE system will be implemented. Remedial measures could incorporate either a continuous or pulsed operation of the SVE system and will be evaluated and discussed with the San Joaquin County Environmental Health Department prior to any further changes. URS proposes that the system be shut off after the May 2004 samples are collected. URS Corporation Crown Corporate Center 2870 Gateway Oaks Drive, Suite 300 Sacramento, CA 95833 Tel: 916.679.2000 Fax: 916.679.2900