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EEC S 1462 01 UnlFirst Stockton September 24,2004 2 2 Physiography The subject site is located in Township 1 North, Range 6 East, in the City of Stockton, County of San Joaquin, California(Figure 1) The most recent topographic map coverage of the site vicinity is provided by the United States Geological Survey (USGS) 7 5 minute Stockton West quadrangle, dated 1968, with photo revisions in 1987 The site elevation is approximately 15 feet above mean sea level (MSL) 2.3 Geologic and Hydrogeologic Setting This site is located in the northern part of the San Joaquin Valley The valley fill consists of late 1 Mesozoic through Eocene sedimentary deposits, predominately of marine origin These deposits are overlain by Tertiary sedimentary deposits which are composed primarily of sand, silt, gravel, and clay (CDMG, 1976) In the vicinity of the site, the late Cenozoic deposits are approximately 3,000 feet thick and generally consist of alluvial gravel, sand, silt, and clay Groundwater in ' these alluvial deposits is usually confined to semi-confined Groundwater beneath the site is located approximately 30 feet below ground surface (bgs) This estimated groundwater depth is based on a depth to groundwater measurement made by EEC in August 2002 at a service station located approximately 1-112 blocks southeast of the subject site 3.0 PRE-UST DISCOVERY ACTIVITIES On November 5, 2003, during site redevelopment grading activities, UniFirst's general contractor, Colton Construction (Colton), encountered hydrocarbon-impacted soil during grading ' activities At the request of UniFirst, EEC mobilized to the subject site to determine the extent and origin of the impacted soil In accordance with the oversight agreement between UmFirst and SJCEHD, dated August 2002, which designates SJCEHD as the local oversight agency for site investigation activities, SJCEHD was verbally notified on November 11, 2003, that hydrocarbon-impacted soil was encountered at a new area of the site EEC collected surface soil samples and advanced direct push (Geoprobe) boreholes to delineate the extent of the hydrocarbon-impacted soil The Geoprobe activities were conducted in accordance with the permits from the SJCEHD Assessment activities are discussed in more detail below 3.1 Surface Sample Collection ' On November 6, 2003, EEC initiated assessment of the origin of hydrocarbon-impacted soil encountered during grading activities The collection of the surface samples was requested by UmFirst to evaluate the nature of the contaminants encountered by the construction workers Visual observations and a Photovac Model 2020 photo-ionization detector (PID) were used initially to evaluate the area of impacted soil Field observations and PID readings indicated that the impacted area was located in the northwest corner of the current construction area (southwest corner of the subject site) The soil did not appear to have any visible staining, but did have a strong chemical odor which increased with depth initial classification of the soil was conducted by collecting a total of six surface samples (SS-1 to SS-6) from the northwest corner of the construction area (Figure 3) Each sample was collected using a 6-inch stainless steel sampling sleeve, capped, labeled, placed on ice, and transported to 3