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1 Jonas & Associates Inc On November 2, 1987, a 1,500-gallon, single-walled UST and associated dispenser and piping were removed from the facility The UST was approximately 25 years old While in service, the tank had stored both leaded and unleaded gasoline Figure 1-2 identifies the former UST location The dispenser was located directly over the tank � �'-` ^------� ' Location of leak: It is most likely that the valve assembly on the UST was leaky The UST was found to be tight when excavated How leak was discovered: On October 2, 1986, a tank-only precision test was performed on the tank The tank failed the test, and was emptied on October 3, ' 1986 Estimate of volume of release: It was estimated from subsurface data collected in ' 1987 and 1988 that approximately 22 gallons of gasoline were present in the subsurface at that time Duration of leak: Unknown Effectiveness of leak detection monitoring program: Precision testing proved to be effective in identifying leakage Pollutants involved: Total Petroleum Hydrocarbons as gasoline (TPH-G), benzene, ' toluene, ethylbenzene, and xylenes (BTEX), and methyl tert-butyl ether (MTBE) 2.0 INVESTIGATIVE METHODS Site investigation methods included a soil vapor assessment, collection of soil ' samples, and installation, development, and quarterly sampling of groundwater monitoring wells from April 1989 to November 1995 Presented below is a summary of investigative methods ' 21 Soil Vapor Assessment ' In 1987, PG&E retained EA Engineering, Science, and Technology, Inc (EA), to carry out a Soil Vapor Assessment (SVA) in the vicinity of the UST ' Soil vapor samples were collected from driven vapor points at 13 locations (see Figure 3-2) Prior to sample collection, a vacuum pump was used to purge the vapor from the probes, to ensure that any vapor samples collected were 2 r r