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ERI 227903 R02 Former Unocal Service Station 5098,Stockton,California July 30, 1999 BACKGROUND The former Unocal service station site, presently occupied by Washington Mutual Bank and Icon Business Solutions, is located on the northeastern corner of Pacific Avenue and Robinhood Drive in Stockton, California, as depicted on the Site Vicinity Map (Plate 1) The present building configuration, former service station layout, monitoring well locations, and other selected site and vicinity features are shown on the Map of Area (Plate 2) Properties in the vicinity of the site are ' occupied by commercial developments The groundwater surface in the vicinity of the site has fluctuated from approximately 41 to 61 feet below grade in existing wells with a general rising trend over the past several years Based on review of the boring logs, the strata beneath the site are in hydraulic equilibrium and comprise a single water- bearing zone The groundwater flow direction appears to be east to southeast during the winter months L with a hydraulic gradient ranging from approximately 0 002 to 0 007, and south to southwest during summer months with a hydraulic gradient ranging from approximately 0 006 to 0 015 (ERI, 1998) Status of Re�siduaUetroleum Hydrocarbons in Soil During September to December of 1988, Unocal excavated soil containing petroleum hydrocarbons to a maximum depth of 53 feet below grade (approximate depth of groundwater during excavation activities) or to the vertical extent of residual petroleum hydrocarbons Based on current water level measurements in the nearest wells to the excavation, the bottom of the excavation extends approximately 12 feet below the current water level Approximately 6,000 cubic yards of soil were �l removed from the site (Unocal, 1996) Based on the results of remedial efforts to remove soil containing petroleum hydrocarbons, the Central Valley Regional Water Quality Control Board (Regional Board) provided a letter, dated August 25, 1989, which "confirmed the soil contamination at the subject site has been adequately cleaned up" Delta College Irrigation Well An irrigation well, utilized by Delta College, is located approximately 350 feet southwest of the site as shown on Plate 2 According to the Water Well Drillers Report, the well was constructed in 1950 to a total depth of 325 feet with perforations beginning at 40 feet below grade Based on conversations with Delta College representatives, the well is typically pumped between the months of May through October at a rate of approximately 240,000 gallons per night (7 00 p ni to 6 00 a in ) Groundwater samples for laboratory analysis have been collected from the well and analyzed by Unocal and Montgomery Wards The most recent analyses detected total purgeable petroleum hydrocarbons as gasoline (TPPHg) at a concentration of 130 parts per billion (ppb) and benzene at a concentration of 28 ' ppb Methyl tertiary butyl ether (MTBE) was not detected at or above laboratory method detection limits in the sample collected from the irrigation well 2