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Chevron Products Company age 2 \1) Chevron Service Station 9 4275 2905$enjannn Holt Drive Stockton California May 20 02 SITE BACKGROUND Chevron 9-4275 is an active service station located at the northwest corner of Benjamin Holt Drive and Plymouth Road in Stockton, California Current station features include four double- walled, fiberglass USTs in a common excavation, a station building, six product dispensers under a common canopy, and a car wash The site is located in a light commercial/residential area of Stockton in a relatively flat lying area A summary of previous investigations pertinent to the assessment is located in Attachment A ASSESSMENT ACTIVITIES AND FINDINGS Assessment activities were performed concurrently with monthly sampling of Municipal Well No 10, from March 2001 through August 2001 Monthly sampling of Municipal Well No 10 was initiated in response to an unqualified MtBE detection in this well during the 4th quarter 2000 routine groundwater monitoring for the site The following activities were completed as part of the assessment Literature Review The City of Stockton Public Works Department provided a copy of the Water Well Drillers report for Municipal Well No 10 to aid in designing the investigation Review of the report in Attachment B, suggests the following • The sanitary seal for the well was placed from surface to 55- feet bgs, completed within a . clay layer encountered from 48- to 72- feet bgs, o A coarse sand layer, from 72- to 79 feet bgs lies between the sanitary sealing clay layer and a deeper clay-confining layer extending from 79- to 146- feet bgs, o Municipal well No 10 is screened at two depth intervals of 295- to 315- feet bgs and 333- to 353- feet bgs The water quality within the 72- to 79-foot depth interval represented a data gap Detections of any chemical of concern in this stratum could potentially establish a conduit from the upper affected water-bearing zone to the borehole of Municipal Well No 10 The field data collection described below was designed to measure MtBE in this stratum Field Activities ® CPT Investigation On May 18, and 21, 2001, SECOR supervised Gregg Drilling &Testing, Inc (Gregg) of Martinez, California, as they advanced five Cone Penetrometer Tests (CPT -1 through CPT-5) along the eastern side of Plymouth Road to assess soil stratigraphy and collect discrete groundwater samples (Figure 2) A second proposed 5-CPT transect along the western side of Plymouth Road could not be completed due to an underground utility corridor Each CPT boring was monitored continuously for soil behavior and pore pressure to a depth of approximately 100 feet below ground surface (bgs) The depth profile was selected to . confirm lithology described in the City of Stockton water well drillers report Based on the I 1Chevron1942751REPORTS1200I CPT SA and Muni sampling Rev A doe