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Atlantic Richfield Company(ARCO) 3 ARCO Station #4932 16 E Harding Way,Stockton CA January 15 2002 • Over-excavation of the UST complex was completed and samples were collected on June 9, 1999 Six samples collected from the excavation at approximately 19 feet below ground surface (bgs) One sample contained total petroleum hydrocarbons as gasoline (TPHg) and methyl tert-butyl ether (MtBE) at concentrations of 1 02 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 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 1n two samples at a maximum concentration of 55 5 mg/kg This work was summarized in SECOR's report titled Soil Sampling During USTs, Product Line, 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 vapor extraction well (SVE-1) were installed at the site Groundwater samples collected from these wells were reported to contain TPHg, benzene, and MtBE and MW- 4 contained maximum concentrations of 166,000 micrograms per Liter(ug/1), 10,500ug11, and 708,000 ugll , respectively Other detected constituents included tert-amyl methyl ether(TAME), 1, 2 dichloroethane (1,2 DCA), tert-butyl alcohol TBA and ethyl tert-butyl ether(EtBE) This work was summarized in SECOR's report titled Site Assessment Report dated November 6, 2000 Routine groundwater monitoring continues to suggest these constituents are present at actionable levels and separate phase hydrocarbons (SPH) have been detected in wells MW-4 and SVE-1 Groundwater flow direction beneath the site has been consistently mapped to the northeast at gradient ranging from 0 004 to 0 04 ft/ft During February 2001, SECOR performed multi-phase extraction (MPE) testing at the site to assess WE technology as a feasible remedial alternative for mitigating hydrocarbon-impacted soil The results of the feasibility test are as follows • The five-day MPE test produced vapor extraction flow rates ranging between 32 7 to 99 2 standard cubic feet per minute (scfm), from MW-4 and SVE-1, and at vacuum pressures ranging between 5 5 to 25 inches of water column from MW-4 and SVE-1 • Based on the responding induced vacuums up to 89 feet away, the calculated radii of influence for the test ranged between 46 7 feet and 89 2 feet • During the test approximately 544 3 pounds of TPHg, 19 5 pounds of benzene, and 440 pounds of MtBE were extracted from the soil beneath the site, as calculated from field FID readings that consistently exceeded 10,000 parts per million by volume (ppmv) and laboratory results i WKPLAN4932—1-11 02 doe