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URS ISeptember 19, 2003 . Page 3 I The geology of the Sacramento Valley consists mainly of Holocene to younger Oligocene continental fsedimentary and volcanic deposits These deposits unconformably overlie thick sequences of inter- fingered Eocene to Cretaceous sediments The Eocene to Cretaceous sediments unconformably overlie the Mesozoic Intrusive felsic (granitic) basement rocks of the Sierra Nevada Batholith (and associated Paleozoic metasediments and metavolcanics) 1.4 Site Geology and Hydrogeology IBased on observations during drilling activities, the site vicinity is underlain primarily by silt with inter- bedded clay The silty zone is underlain by a sandy zone composed of sand and silty sand A second I silty zone is beneath the sandy zone and a second sandy zone is beneath the second silty zone Boring logs are provided In Attachment A 'The site geology is graphically presented in two geologic cross sections from Delta's Groundwater Assessment Report (Delta 2001), see Attachment B A site map showing the section locations is provided with the cross sections Depth-to-water measurements at the site have ranged from approximately 34 to 59 feet below ground surface (bgs) (Table 1) Monitoring data from June 9, 2003 indicates that the dominant inferred groundwater flow directions are east-northeast, with average hydraulic gradients of 0 01 feet per foot A groundwater elevation contour map is provided as Figure 3 Historical data shows groundwater direction I. and hydraulic gradient to be variable The direction of flow and gradient will change between sampling Intervals 1.5 Well Search A water supply well owned by the City of Stockton was identified approximately 1100 feet west of the site In 1998 This well was reportedly situated in Oak Park(Delta, 2001) ARCO contracted IT to perform a well survey identifying water supply wells in the vicinity of the station The survey identified three wells within 2,000 feet of the site Two wells, one approximately 900 feet to the north of the site, the other approximately 400 feet to the west-northwest, are owned by the City of Stockton and are used to irrigate Oak Park The third well is approximately 1,800 feet from the site also to the west-northwest and is located on the property of St Mark's Church The well on the ' St Mark 's property corresponds with the location and owner of the well identified in the Pinnacle survey but could not be field verified (Delta, 200 1) 2.0 EXTENT OF PETROLEUM HYDROCARBON IMPACTS The extent of hydrocarbon impacts at the site Is based on information contained in the Quarterly ' Groundwater Monitoring Report Second Quarter 2003 (URS, 2003), Underground Storage Tank and Product Delivery Lane Removal and Replacement for ARCO Products Company Facility No 2186 letter to Ms Carol Oz (Pinnacle, 1999) and the Groundwater Assessment Report, ARCO Facility No 2186 ' (Delta, 200 1) K\WprocessU5 RP ARC01021861CAP Aug03\02186 CAP03 doe