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ERI <br /> ENVIRON ENTAL RESOLUTIONS, INC. <br /> January 29, 1999 <br /> ERI 205513 QS7 <br /> Mr Ed Ralston <br /> Tosca Marketing Company <br /> 1380 Lead Hill Road, Suite 120 <br /> Roseville, California 95661 <br /> Subject Quarterly Summary Report for Environmental Investigation, Fourth Quarter 1998, <br /> Tosco 76 Service Station 5886, 2701 March Lane, Stockton, California <br /> Mr Ralston <br /> kt the request of Tosco Marketing Company (Tosco), and pursuant to correspondence from San <br /> Joaquin County Public Health Services, Environmental Health Division (the County) dated July 24, <br /> 1997, Environmental Resolutions, Inc (ERI) is submitting this quarterly summary report for the <br /> subject site <br /> BACKGROUND <br /> The site is immediately west of Interstate 1-5 on the northwestern corner of March Lane and the I-5 <br /> offramp in Stockton, California, as shown on the Site Vicinity Map (Plate I) The locations of the <br /> existing underground storage tank (UST) field, dispenser pump islands, other selected site features, <br /> and surrounding properties are shown on the Generalized Site Plan (Plate 2) Properties in the <br /> vicinity of the site are generally occupied by commercial developments <br /> It is ERI's understanding that Unocal Corporation (Unocal) purchased the land in 1967, at which time <br /> the property was vacant In 1982, Unocal constructed a service station at the site and installed three <br /> 12,000-gallon UST's to store gasoline and diesel fuels (Delta Environmental, February 6, 1996) <br /> Between July 1995 and September 1996, Unocal replaced the original UST's and associated product <br /> distribution lines, and enlarged the existing tank pit to accommodate three new 15,000-gallon UST's <br /> Unocal also advanced 21 soil borings and collected soil and groundwater samples from the borings <br /> utilizing a Geoprobe"O sampling system, drilled four additional soil borings, and constructed <br /> groundwater monitoring wells in three of these borings The location of existing wells are shown on <br /> Plate 2 <br /> Unocal initiated quarterly groundwater monitoring during December 1996 Data collected by others <br /> indicate the groundwater flow direction ranges from northwest to southwest and the groundwater <br /> surface ranges from approximately 5 to 9 feet below ground surface (ft bgs) <br /> In April 1997, ERI performed an investigation to evaluate the source and extent of methyl tertiary <br /> butyl ether (MTBE) (ERI, June 3, 1997) The investigation included hand-auguring three soil borings <br /> • adjacent to catch basins, drilling four soil borings, and constructing dual completion groundwater <br /> 74 Digital Drive, Suite 6, Novato, California 94949 415-382-9105 (FAX 415-382-1856) <br /> Lake Forest • Novato • Seattle <br />