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Ms Margaret Lagorio Project 20805-152 008 October 12, 1998 Page 2 BACKGROUND The site is an active service station Four underground storage tanks (USTs) are currently in service for storage of regular grade, mid grade and premium grade unleaded gasoline RESNA drilled five soil borings in June 1990 in the vicinity of the UST complex to assess the lateral and vertical extent of petroleum hydrocarbon impacts to soil prior to UST removal Petroleum hydrocarbons were detected in soil samples collected from the borings In November, 1989, Roux observed the removal of five UST and associated piping and collected compliance soil samples from the UST cavity and piping trenches Four new USTs were then installed in an excavation located adjacent to the former UST complex Between October 1991 and December 1992, RESNA drilled 21 soil borings and installed 16 vapor extraction wells at the site Petroleum hydrocarbons were detected in soil beneath the dispenser islands, between the USTs and the dispensers, to the north and west of the former UST location, and near the southeast corner of the site Evaluation of soil sample analytical results showed that with the exception of the southeast part of the site the lateral and vertical extent of impacts was delineated The majority of the residual petroleum hydrocarbon impacts to soil extended to depths of 50 to 70 feet bgs In the vicinity of VW-17, impacts extended to 85 5 feet bgs RESNA evaluated Department of Water Resources (DWR) records for information regarding the on-site water supply well DWR records show that the well was installed in June 1969 and the first-encountered saturated unit was located at 396 feet bgs Perforated intervals are located at 396 to 416 feet bgs and at 486 to 536 bgs After development of the well, water was measured at 100 feet bgs, suggesting groundwater is under confined conditions Vapor extraction wells drilled to depths of 115 feet bgs did not encounter groundwater In 1995 EMCON conducted additional assessment southeast of the site to further define the lateral and vertical extent of impacts Low concentrations of TPHG and BTEX were detected at depths between 45 and 65 feet bgs The vertical extent of impacts was defined Soil types encountered during site assessment activities consisted primarily of silt and clay with thin interbedded sand to silty sand lenses Sand units do not appear to be laterally or vertically continuous SACK\5G12NDQTR--116100MASS DOC-941mn 1