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S E C 0 R 7-Eleven Store#14117 Work Plan for Additional Site Assessment July 27, 2004 Page 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 sod bonngs The offsite borings did not contain total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and total xylenes (BTEX), or MtBE in soil above laboratory reporting limits (Table 1), except for one detection of total xylenes in the shallow sample (4 foot below ground surface) in OSGP-2 (Table 1) The onsite bonngs reported benzene, TPHg, and 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) TBA was detected at two samples at a concentration of 166 mg/kg (GP-14@10') and 23 4 mg/kg (GP-17@15') On May 13 and 14, 1999, four groundwater monitoring wells (MW-1 through MW-4) were advanced to 51 feet below ground surface (bgs) 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 1 95 mg/kg (MW-3-15', Method 8020), respectively (IT Corporation, 1999) (Table 1) On October 30, 2001, three groundwater monitoring wells (MW-5, MW-6, and MW-7) were advanced 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') (Table 1) Selected halocarbons also reported in the soil samples indicated possible dry cleaner plume contributions (IT Corporation, 1999) An active USA Gasoline (former Exxon) service station is located east of the site On August 14, 2003 SECOR oversaw the installation of one monitoring well (MW-8) and one soil boring (SBA) (Figure 2) Soil samples collected from MW-8 contained petroleum hydrocarbons to 50 feet bgs Grab groundwater samples collected from SBA contained TPHg, MtBE, and TBA as deep as 52 feet bgs (Table 2) On February 27, 2004 SECOR submitted the Additional Site Assessment Report and Fate and Transport Model, which included the results of the August 2003 well installation and the results of Fate and Transport modeling using Flowpath 2 Historical groundwater flow direction data indicated a time-dependent variability in groundwater flow direction, which appeared to result in a net zero gradient over an extended period of time Four models were run with varying results the first and second modeling runs were very similar and indicate that MtBE will migrate approximately 120 to 180 feet from the source area to the northeast, the third and fourth modeling runs represented a near-"flat" hydraulic gradient and indicated that the MtBE contamination would remain in the immediate vicinity of the UST area and concentrations will decrease with time due to dispersion and natural decay Overall, it was determined that it is difficult to accurately model the migration of contaminant plumes at this site because of the relatively flat groundwater table and the highly variable groundwater flow direction in the area • rale 7-11114117 WP for additional assessment(July 04)doc SECOR Internatronat Incorporated