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Atlantic Richfield Company Page 3 ARCO Station No 4932 Stockton California August 29,2002 Routine groundwater monitoring continues to suggest these constituents are present at actionable levels and LPH has 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 DPE testing at the site to assess DPE technology as a feasible remedial alternative for mitigating hydrocarbon-impacted soil The results of the feasibility test are as follows • The five-day DPE test produced vapor extraction flow rates ranging between 32 7 and 99 2 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 laboratory results and field FID readings that consistently exceeded 10,000 ppmv . The test activities and findings were summarized in SECOR's report titled Interim Remediation/Multi-Phase Extraction Feasibility Test Report dated May 4, 2001 During July 2001,West Hazmat Dnlling (West Hazmat)of Rancho Cordova, California advanced two soil borings and installed five groundwater monitoring wells to further investigate the lateral and vertical extent of petroleum hydrocarbons in soil and groundwater beneath the site Soil borings SB-1 and SB-2 were drilled to 80 feet and 78 feet bgs, respectively Wells MW-5 through MW-9 were completed at 40 feet bgs Results of the investigation indicated that the vertical extent of petroleum hydrocarbons including MtBE in soil was vertically defined beneath the site by deep borings SB-1 and SB-2, and off-site by the borings for wells MW-5, MW-7, and MW-8 The presence of MtBE in the deepest soil sample collected from the boring for well MW-6 (8 2 mg/kg) suggested that additional investigation for vertical off-site delineation was warranted Based on groundwater analytical results, SECOR concluded that the lateral extent of TPHg and benzene had been defined by wells MW-5 through MW-8, however the presence of MtBE in groundwater samples collected from well MW-6 and depth-discrete groundwater samples from boring SB- 2 confirmed that further lateral and vertical delineation of the dissolved MtBE plume was required within these areas SECOR further concluded that the presence of TPHg and benzene in upgradient well MW-9 suggested that an additional source(s) of petroleum hydrocarbons might be present in the area Results of this investigation are documented in SECOR's Additional Site Assessment Report, dated November 9, 2001 1\ARCO\49321PAR\PAR doc