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Apr- 14-98 12 : 34P Pinnacle/EMCON P .02 <br /> ��L iWGGL a division of .aa—I&& <br /> ENVIRONMENTAL SOLUTIONS `mrya• <br /> April 8, 1998. <br /> Project 20805-146.010 <br /> Mr. Ron Rowe <br /> San Joaquin County Public Health Department <br /> Environmental Health Division <br /> P.O. Box 388 <br /> Stockton, California 95201-0388 <br /> Re: Results of Soil Vapor Extraction Assessment, Former ARCO Service Station NO, <br /> 2168, located at 441 West Charter Way, Stockton, California <br /> Dear Mr. Rowe: <br /> EMCON has prepared this letter on behalf of ARCO Products Company (ARCO) <br /> presenting the results from the evaluation of the soil vapor extraction (SVE) system <br /> performance at the Former ARCO Service Station No. 2168, located at 441 West Charter <br /> Way, Stockton, California. The work performed in this letter is based on our telephone <br /> conversation and letter dated January 27, 1998 and February 2, 1998, respectively. <br /> As outlined in our February 2, 1998 letter, EMCON shut off the soil vapor extraction <br /> (SVE) system for a three week period to allow for soil vapor concentrations to rebound. <br /> Commonly, during SVE activities at a site, the concentrations continue to decline during <br /> SVE operation only to rebound during non-operational periods. EMCON has been <br /> operating the system at the site over the past few quarters in an intermittent pulse mode to <br /> account for and treat these episodes of higher concentrations. The purpose of this <br /> evaluation was to (1) quantify the amount of any rebound and if present, (2) assess the <br /> length of time required for it to decline back to pre-shutdown levels. <br /> Previous Operational Data <br /> Prior to shutting down the SVE system to evaluate the response of petroleum hydrocarbon <br /> concentrations in the vadose zone, EMCON collected background influent samples at the <br /> site in January 1998. The analytical results indicated that influent concentrations were <br /> 1,200 part-per-million by volume (ppmv) for total petroleum hydrocarbons as gasoline <br /> (TPHG) and 2.8 ppmv for benzene. Table 1 presents the analytical data for the pre- and <br /> post pulse sampling events. <br /> SACC:\F7LG5 WiCO12168UtOWS2.DOC-9$mtt:l <br /> 3939 Cambridge Road,SU6e 220 Cameron Park.Caffomia 95682 (530)876-8884 (530)678-6885 Fax <br />