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7-Eleven Store #20632 Quarterly Groundwater Monitoring Report (3Q2004) • October 28, 2004 Page 2 Average Change in Groundwater Elevation Since Last Event 0 30 foot increase Groundwater Flow Direction and Gradient Southeast @ 0 002 foot per foot (Fide 21 _ Current Quarter Analytical Data (See Figure 3 and Table 1) Maximum TPHg Concentrations MW-3, 2,200 IL Maximum Benzene Concentrations MW-41R, 380 /L Maximum MtBE Concentrations MW-7, 17,000 pg/L. Maximum TBA Concentrations MW-7, 55,000 pg/L BACKGROUND Initial site assessment activities were executed in accordance with directives from SJCEHD (Fluor Daniel GTI 1997) The assessment concluded that soil impacted with gasoline appeared restricted to a limited area surrounding the underground storage tank (UST) pit and underlying the fuel dispensers The USTs were replaced as part of 1998 upgrading requirements Concurrent with limited overexcavation, several days of high-volume dewatering occurred as part of construction activities, which also served as a short-term • remediation event Additional lateral and vertical assessment of dissolved methyl-tert-butyl ether (MtBE) and hydrocarbons was conducted through 2002 There are currently seven onsite monitoring wells In April 2002, three onsite monitoring wells (MW-4R, MW-6, and MW-7), two dual- nested sparge/vent wells (SPV-1 and SPV-2) and a vertical delineation soil boring (SB-1) were installed at the site On April 22, 2002, a soil vapor extraction (SVE) test was performed at the site Vapor was extracted from MW-41R, MW-7, SPV-1, and SPV-2 Low influent vapor concentrations and poor vacuum response in observation wells were documented during the test An air sparge test in SPV-1 increased the dissolved oxygen concentration groundwater from 0 5 to 2 4 milligrams per liter (mg/L) in SPV-1 In April 2003, 7-Eleven, Inc upgraded the product lines at the site Benzene, total petroleum hydrocarbons as gasoline (TPHg), and MtBE were reported at maximum concentrations of 0 019 milligrams per kilogram (mg/kg), 4 7 mg/kg, and 0 60 mglkg, respectively, in soil samples collected beneath the product lines and fuel dispensers MONITORING AND SAMPLING PROCEDURES The depth to water was measured to within 0 01 feet bgs in monitoring wells MW-1, MW-2, MW-3, MW-41R, MW-5, MW-6, and MW-7 from the top of casing (TOC) using a water level indicator Dissolved oxygen concentrations were also measured in each well using a YSI Model 550A dissolved oxygen meter equipped with a down hole sensor 20632 04-3Q Report doc SECOR International Incorporated