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7-Eleven Store #14117 Quarterly Groundwater Monitoring Report (4Q2006) February 28, 2007 Page 2 Depth to Groundwater (DTW) (Measured Below Top of Casing) 8.49 to 9.81 ft Average Change in Groundwater Elevation Since Last Event: 0.26 foot decrease Groundwater Flow Direction and Gradient: Radially Outward @ East-Southeast @ 0.007 foot per foot and Southwest @ 0.006 foot per foot (Figure 2) Current Quarter Analytical Data (See Figure 3 and Table 1) Maximum TPHg Concentrations MW-3, 4,000 pg/L Maximum Benzene Concentrations MW-3, 6.2 pg/L Maximum MtBE Concentrations MW-8, 91 pg/L Maximum TBA Concentrations MW-4, 200 pg/L BACKGROUND On July 17, 1998, three 10,000-gallon single-walled USTs were removed from the site. The UST excavation was extended to the east, and two new 10,000-gallon double-walled steel USTs were installed. The new tanks were installed in a tank pit located east of the original UST locations. One ten-inch diameter Schedule 40 polyvinylchloride (PVC) conductor casing was installed in the new UST pit between the new tanks for future installation of well MW-2. On September 29 and October 2, 1998, Fluor-Daniel GTI advanced six offsite (OSGP-1 through OSGP-6) and 11 onsite (GP-7 through GP-17 Geoprobe@ soil borings (Figure 4). The offsite borings were located north of the 7-Eleven site in an apartment complex. The offsite borings did not contain detectable concentrations of petroleum hydrocarbons. The onsffe t6nng`s contained-benzene, total petroleum hydrocarbons as gasoline {TPHg), and methyl-tert-butyl ether (MtBE) at maximum concentrations of 12.5 milligrams per kilogram (mg/kg) (GP-17@20'), 42,900 mg/kg (GP-15@15'), and 234 mg/kg (GP-14@10'), respectively (Fluor Daniel GTI, 1998). On May 13 and 14, 1999, four groundwater monitoring wells (MW-1 through MW-4) were installed to 25 and 30 feet below ground surface (bgs) (Table 3). Benzene, TPHg, and MtBE in soil were reported at maximum concentrations of 2.89 mg/kg (MW-3-15'), 429 mg/kg (MW-3-15'), and 0.119 mg/kg (MW-4-20'), respectively (IT Corporation, 1999). On October 30, 2001, three groundwater monitoring wells (MW-5, MW-6, and MW-7) were installed to 20 feet bgs. TPHg, benzene, toluene, ethylbenzene, xylenes (BTEX), and MtBE in soil were not detected above laboratory reporting limits, except for 0.006 mg/kg total xylenes (MW-7@20'). Selected halocarbons also present in the soil samples indicated possible dry cleaner plume contributions (IT Corporation, 1999). 14117 06-4Q Report SECOR International Incorporated