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S ECOR -� Work Plan for Monitoring Well Installation July 31, 2007 Former ARCO Service Station No. 4932 Stockton, California Page 2 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. GRO was reported in three of the six samples collected from beneath the former product lines and dispensers at a maximum concentration of 5,430 mg/kg. Benzene was reported in two samples at a maximum concentration of 10.6 mg/kg. MTBE was reported in two samples at a maximum concentration of 55.5 mg/kg. This work is summarized in SECOR's report titled Soil Sampling During UST's, 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 GRO, benzene, and MTBE. MW-4 contained maximum concentrations of 166,000 micrograms per liter (Ng/L), 10,500 pg/L, and 708,000 pg/L, respectively. Other detected constituents included tert-amyl methyl ether (TAME), 1,2- dichorloethane (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 liquid phase hydrocarbons (LPH) have been periodically detected in monitoring and soil vapor extraction wells MW-4 and SVE-1, respectively. Groundwater flow direction beneath the site has been consistently mapped to the northeast at a gradient ranging from 0.004 to 0.04 feet per foot (ft/ft). During February 2001, SECOR performed dual phase extraction (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 standard cubic feet per meter (scf/m), 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 radius of influence for the test ranged between 46.7 feet and 89.2 feet. • During the test approximately 544.3 pounds of GRO, 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 flame ionization detector readings that consistently exceeded 10,000 part per million by volume (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 Drilling (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 monitoring wells MW-5, MW-7, and MW-8. The presence of MTBE in the deepest soil sample collected from the 4932 WP for MW Installation 2007.doc SECOR International Incorporated