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October 30, 1997 - Page 2 • The residual dissolved hydrocarbon plume is small and stable Separate-phase hydrocarbons (SPH) have never been detected in groundwater monitoring wells The distance to the nearest known municipal water-supply well is approximately 1,800 feet In a letter to San Joaquin County Public Health Services,Environmental Health Division (PHS/EHD) dated September 12, 1997, Tosco concluded that the dissolved hydrocarbon plume appeared stable and the site is potentially suitable for closure as a low-risk groundwater case In a letter dated September 26, 1997, PHS/EHD responded that they could not agree that the plume was stable and that additional assessment work would be required (Attachment B) An addendum work plan was required to be submitted by ' Tosco to PHS/EHD by October 15, 1997 F BACKGROUND The following sections provide a summary of site activities and conditions Text material and illustrations contained in Attachments are taken from reports Iisted under References/Enclosed Reports Site Description (La , Lb.). The active Unocal service station is located at i4707 Pacific Avenue in Stockton, California (Attachment C) The station is located in on a gentle slope at an elevation of approximately l3 feet above mean sea level Proper- ties adjacent to the site are used pnmarily for commercial buildings and parking lots - The station originally contained three 12,000-gallon USTs (see map in Attachment C) In June 1988, the three original USTs were removed and replaced with two double-wall 12,000-gallon USTs storing regular unleaded and super unleaded gasoline In July 1994, the dispenser and piping systems were upgraded to meet 1998 standards The site is currently an active Unocal service station Geologic/Hydrogeologic Setting (l.f.) The site is located in the San Joaquin Valley which is underlain by thousands of feet of alluvial deposits eroded from adjacent high- lands Site bonngs found the station area to be underlain by pnmanly silt with interven- ing layers and lenses of fine to medium sand Mammum depth explored at the site was 72 5 feet below ground surface (bgs) A set of geologic cross-sections is presented in Attachment D In February 1992, groundwater was initially found beneath the site at a depth of approximately 50 feet bgs By September 1992, groundwater levels had dropped to a depth of approximately 56 feet bgs Since late 1992, groundwater levels in site wells • 3111657A1CL6981 DOC