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If k 4PC AM B R I A Previous Investigations- During October 1988, two dispensers were removed, and soil impacted by petroleum hydrocarbons beneath the eastern dispenser was excavated to an approximate depth of 15 feet below grade(fbg) Soil collected from the bottom of the excavation contained total petroleum hydrocarbons as gasoline (TPHg) at concentrations ranging from 3 to 5,300 parts per million (ppm) and benzene at concentrations up to 31 ppm This investigation is described in the October 9, 1990 Environmental Assessment Status Report written by Groundwater Technology,Inc (GTI)of West Sacramento, California During May 1989, GTI drilled four vertical soil borings and one angle soil boring onsite From May 1989 through May 1990, GTI installed seven onsite groundwater monitoring wells and four offsite monitoring wells Maximum concentrations detected in soil from these borings were 2,500 ppm TPHg and 11 ppm benzene These activities are also summarized in GTI's October 9, 1990 Environmental Assessment Status Report During April 1992, GTI installed four soil vapor extraction (SVE) wells and conducted an SVE pilot test These results are described in GTI's September 16, 1992 Summary Report of Soil Vapor Extraction Test Sampling Activities During September 1993, GTI installed five offsite groundwater monitoring wells Soil from these borings contained TPHg at concentrations up to 3 7 ppm and benzene at concentrations up to 0 23 ppm. This investigation is documented in GTI's December 16, 1993 Summary Report of Drilling Activities Separate phase hydrocarbons (SPH) had historically been detected in well MW-1, but no measurable SPH has been observed since',March 1993 From May 1989 to December 1989, approximately 45 gallons of SPH were recovered from well MW-1 An SVE test was conducted in June 1992, and an SVE system began operation at the site in August 1995 SECOR International, Inc of Huntington Beach, California detail this in their September 12, 1996 Soil Vapor Extraction System Installation and Start-up Report Between January 1996 and March 1998, the SVE system removed approximately 6,250 pounds of TPHg The SVE system was shut down March 9, 1998 due to declining influent concentrations and rising groundwater During June 1999, K E Curtis Construction Company removed one 12,000-gallon diesel and three 10,000-gallon gasoline USTs, four dispenser pumps, and their associated piping During August 1999, the soil beneath the UST, piping, and dispensers was further overexcavated This investigation is documented in our October 22, 1999 Underground Storage Tank Removal Report 1399 2