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Atlantic Richfield Company 2 ARCO 4932 16 E Harding Way,Stockton,CA - June 7,2002 SITE BACKGROUND AND PREVIOUS INVESTIGATIONS ' The site is an operating AM/PM mini-mart and Atlantic Richfield Company service station located at the intersection of Center Street and Harding Way in Stockton, California The site is located in a mixed residential and commercial area of Stockton ' Facilities at the site currently include one 20,000-gallon gasoline underground storage tank (UST), and one 22,000 gasoline UST, and associated product lines and dispensers Recent facility upgrades included excavation and removal of three existing gasoline USTs, and associated dispensers and product lines Between May 25 and June 9, 1999, Gettler-Ryan, Inc (GRI) of Dublin, California removed three 10,000-gallon double- walled fiberglass USTs and enlarged the excavation to accommodate the larger new USTs Approximately 631 tons of soil was over-excavated from the UST complex Over-excavation of the UST complex was completed and samples were collected on June 9, 1999 Six samples were collected from the excavation at approximately 19 feet below ground surface (bgs) One sample contained total petroleum hydrocarbons as ' gasoline (TPHg) and methyl tertiary butyl ether (MtBE) at concentrations of 102 milligrams per kilogram (mg/kg) and 0 579 mg/kg, respectively No benzene or fuel oxygenates were detected in any other samples from the over-excavation During the ' facility upgrade, approximately 150 tons of soil was over-excavated from the product line and dispenser areas (Figure 2) TPHg was reported in three of the six samples ®r)' collected from beneath the former facilities at a maximum concentration of 5,430 mg/kg Benzene was reported in two samples at a maximum concentration 10 6 mg/kg MtBE was reported in two samples at a maximum concentration of 55 5 m.&g This t work is summarized in SECOR's report titled Soil Sampling Durtng USTs, Product Lane, and Dispenser Removal Activities dated December 10, 1999 In August 2000, four groundwater monitoring wells (MW-1, MW-2, MW-3, and MW- 4)and one soil vapor extraction (SVE) well (SVE-1) were installed at the site Groundwater samples collected from these wells contained TPHg, benzene, and MtBE t Maximum concentrations of 166,000 micrograms per liter(ugll) TPHg, 10,500 ug/l benzene, and 708,000 ug/1 MtBE were reportedly detected in well MW-4 Other detected constituents included tertiary amyl methyl ether (TAME), 1, 2-dichloroethane (1,2-DCA), tertiary butyl alcohol (TBA), and ethyl tertiary butyl ether (EtBE) This work is summarized in SECOR's report titled Site Assessment Report dated November 6, 2000 During February 2001, SECOR performed multi-phase extraction (MPE) testing at the ' site to assess whether MPE technology is a feasible remedial alternative for mitigating petroleum hydrocarbon-impacted soil The results of feasibility testing are as follows • The five-day MPE test performed utilizing wells MW-4 and SVE-1 ' produced vapor extraction flow rates between 32 7 and 99 2 standard cubic •, feet per minute (scf:m) at vacuum pressures ranging between 5 5 to 25 inches of water column I I 1\ARCO149321ReportslAsmt&AgfrTstRpt doc