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[ C 0 R 7-Eleven Store#14117 Quarterly Groundwater Monitoring Report (1Q2005) May 24, 2005 • Page 2 Average Change to Groundwater Elevation Since Last Event 1 44 foot increase Groundwater Flow Direction and Gradient Northeast @ 0 01 feet per foot (Figure 2 Current Quarter Analytical Data (See Figure 3 and Table 1) Maximum TPHg Concentrations MW-3, 5,100 IL Maximum Benzene Concentrations MW-2, 760 IL Maximum MtBE Concentrations MW-2, 150 }ag/L _ Maximum TBA Concentrations MW-1, 2,000 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 to 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 onsite borings 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) On August 14, 2003, deep groundwater monitoring well (MW-8) and one soil boring (SB-1) were advanced onsite SECOR summarized this assessment in the February 27, 2004 Additional Site Assessment Report and Fate and Transport Model, which also presented calculations of the lateral migration of dissolved MtBE in groundwater • I 14117 05-1 Q Report doc SECOR Intemational Incorporated