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ECOR 7-Eleven Store #14117 Quarterly Groundwater Monitoring Report (3Q2004) November 1, 2004 Page 2 Historic Range in Depth to Water, Q3-99 to Q3-04 MW-1, 6 69 to 10 52 feet bgs Current Quarter Monitoring Data (See Figure 2 and Table 1) Wells Monitored and Sampled Eight wells, MW-1 through MW-8 Dissolved Oxygen Concentrations Measured In Eight wells, MW-1 through MW-8 Depth to Groundwater (DTW) 8 89 to 10 52 ft bgs Average Change to Groundwater Elevation Since __... Last Event 1 75 foot decrease Groundwater Flow Direction and Gradient Southeast @ 0 01 feet per foot and Northwest @ 0 02 feet per foot (Figure 2) Current Quarter Analytical Data (See Figure 3 and Table 1) Maximum TPHg Concentrations MW-3, 9,500 /L Maximum Benzene Concentrations MW-8, 32 pg/L _ Maximum MtBE Concentrations MW-8, 61 pg/L Maximum TBA Concentrations MW-1, 3,100 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 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, 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 14117 04-30 Report doe SECOR Intemationa!Incorporated