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f S E C 0 R • 7-Eleven Store Number 19976 Fuel System Upgrade Report August 7, 2003 Page 3 limited screen above the water table likely account for low extraction rates/low induced vacuum responses observed during extraction from the wells Extracted vapor concentrations ranged up to 0 91 ppmv benzene, 220 ppmv MtBE, and 880 ppmv TPHg The data indicated that vapor extraction is a viable remediation method at the site, particularly when the horizontal wells are used Double-nested sparge well SP-1 was installed on October 29-30, 2001 to a depth of 50 feet bgs and perforated at 30-35 and 45-50 feet On February 6, 2002, an air sparge test was conducted using well MW-2 for vapor extraction, and double-nested sparge well SP-1 for air injection Pre-sparging extracted vapor levels ranged up to 1,000 ppmv, with dissolved oxygen (DO) levels of up to 0 72 mg/L Air was then injected into the deeper section of well SPA using an oil-less air compressor Vapor levels in well MW-2 increased to 1,600 ppmv, and DO levels to 8 milligrams per liter (mg1L), after 30 minutes of sparging The data indicated the area of sparging influence in SP-1 includes the region of MW-2, and that treatment using well SPA should be successful at reducing dissolved hydrocarbons/oxygenates in well MW-2 and the surrounding area Air sparge testing was repeated on May 23, 2002, using well MWAA as the injection point and the horizontal wells as the SVE point Results indicated no breakthrough of sparged air into the vadose zone accessible by the horizontal wells QSITE LOCATION AND FACILITY DESCRIPTION The site is located at the southwest corner North Main Street and Northgate Drive in Manteca, California Access to the property is provided by North Main Street to the east and Northgate Drive to the north The site is located In a mixed residential and commercial area The site is occupied by an operating 7-Eleven convenience store, which is located along the western portion of the property Before replacement, three USTs were located in the central portion of the property between the 7-Eleven store and the approach on North Main Street (Figure 1) The concrete drive slab and two dispenser islands are located adjacent and north of the USTs The dispenser islands and UST areas are paved with concrete and the remainder of the site is surfaced with asphalt The dispenser island and UST concrete pads were removed during the fuel system upgrade activities FIELD ACTIVITES Sample Collection and Analyses — Dispensers and Fuel Lines On May 27, 2003, SECOR's field geologist collected a total of two soil samples in native soil One sample was collected under the northern fuel dispenser (D1-4 5), and the second sample was collected beneath the product line (L14 0) A soil sample was unable to be collected from beneath the southern dispenser due to an excessive thickness of pea gravel Each soil sample was screened for hydrocarbon vapors using a portable photoionization detector(PID) Soil samples were collected in clean, 2-inch diameter by 6-inch-long brass sample tubes A SECOR scientist used a hand auger to extract soil, and then pounded a sample tube into the soil in the center of the auger The sample tube was filled completely with soil to minimize headspace and seated using Teflon sheets and I• plastic end caps Each soil sample was labeled and placed in a cooler on ice for delivery to Kiff Analytical in Davis, California All soil sampling equipment was cleaned with a non-phosphate 19976 Fuel Line&USTs Upgrade Reportl doc SECOR International Incorporated