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E C 0R 7-Eleven Store#14117 Quarterly Groundwater Monitoring Report (2Q2005) August 5, 2005 Page 2 Depth to Groundwater (DTW) 6 00 to 8 29 ft bgs _ Average Change to Groundwater Elevation Since ' Last Event 0 83 foot increase Groundwater Flow Direction and Gradient Northeast @ 0 01 feet per foot (Figure -- Current Quarter Analytical Data (See Figure 3 and Table 1) Maximum TPHg Concentrations MW-3, 5,600 /L Maximum Benzene Concentrations MW-8, 25 pg/L _ Maximum MtBE Concentrations MW-8, 76 pg/L _ Maximum TBA Concentrations _ MW-2, 1,900 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 polyvinylchlonde (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 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 14117 05-2Q Report doc SECOR International Incorporated