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Project No S8100-06-11B <br /> April 19, 1996 <br /> ' SITE INVESTIGATION REPORT <br /> ' 10 INTRODUCTION <br /> 11 General Objectives <br /> The objective of the Task Order was to provide additional information regarding the extent of <br /> petroleum hydrocarbon soil and groundwater impacts beneath the subject site The assessment work <br /> was conducted to attempt to delineate the horizontal extent of total petroleum hydrocarbons as <br /> gasoline (TPHg) and benzene impacts in groundwater and to identify the location of the central <br /> portion of the vadose zone soil plume <br /> The site investigation work completed by Geocon was performed in general accordance with the Site <br /> Investigation Workplan dated January 8, 1996 The requested scope of services included the <br /> installation of four perimeter groundwater monitoring wells in an to attempt to define the lateral limits <br /> of dissolved TPHg in groundwater, the installation of a vapor extraction well adjacent to the tank <br /> ' cavity and former dispenser island in order to evaluate the maximum TPHg concentrations in vadose <br /> zone soil, and the advancement of one soil boring within the former pump dispenser island <br /> 12 Project Location <br /> The project site consists of the Caltrans District 10 Stockton Yard facility located at 1604 South B <br /> ' Street in Stockton, California The 4 acre site is part of Caltrans' District 10 Headquarters complex <br /> and is utilized as an equipment maintenance and storage facility The approximate site location is <br /> depicted on the Vicinity Map,Figure 1 The project boundaries are shown on the Site Plan,Figure 2 <br /> 1 1 3 Previous Investigations <br /> The Stockton Yard was built in approximately 1955 and utilized a fueling station located within the <br /> northwest portion of the site It is understood that one 18,925-liter(5,000-gallon) diesel underground <br /> storage tank (UST), one 37,850-liter (10,000-gallon) unleaded gasoline UST, two pump islands and <br /> ' associated product piping were removed from the site in 1989 The results of laboratory tests of soil <br /> samples obtained from the UST excavations indicated the presence of benzene, toluene, ethylbenzene <br /> and total xylenes (BTEX) at concentrations up to 120 milligrams per kilograms (mg/kg) and <br /> maximum TPHg and total petroleum hydrocarbons as diesel (TPHd) concentrations of 2,000 and 58 <br /> mg/kg, respectively The TPHg concentration of 2,000 mg/kg was reported for a soil sample obtained <br /> at a depth of 4 3 meters (14 feet) below ground surface (bgs) from the fill riser end (west end) of the <br /> gasoline UST <br /> ' 1 <br />