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7.0 SUMMARY AND DISCUSSION 71 Summary ® Subsurface petroleum hydrocarbon impact was discovered at the site in 1991, while the site operated as an Exxon service station Exxon removed the former USTs and product delivery system in March 1994 o TPHG and benzene were reported at maximum concentrations of 880 mg/Kg and 3 5 mg/Kg for soil samples collected at the time of the 1994 UST removal Maximum concentrations of TPHG (25,000 µg/L) and benzene (500 µg/L) in groundwater were reported for a grab groundwater sample collected during excavation of the current UST pit in 1995 This groundwater grab sample was collected near the location of future well VEW-3 o An estimated 762 cubic yards of soil were excavated and removed from the site at the time of UST and fuel dispenser replacement activities in December 1995 o Depth to groundwater beneath the site has increased from approximately 17 to 6 feet bgs (well MW-1) since the initial groundwater monitoring event in August 1991 Northeast and southeast groundwater flow directions have predominately been observed, at consistently low groundwater gradients Given the observed groundwater flow direction, 7-11 facility#14117 appears to be upgradient of the former Exxon/Palisades site o There are no groundwater monitoring wells situated in the areas of highest petroleum hydrocarbon impact to soil Increased groundwater levels have allowed for the sampling of well VEW-3 since 2001 o Relatively high concentrations of MTBE (up to 54,000 pg/L) and TBA (up to 280,000 µg/L) have been reported in well VEW-3 High concentrations of MTBE (up to 3,400 pg/L) and TBA (up to 270,000 pg/L) were also reported in shallow groundwater samples collected at boring locations CPT-2 and CPT-3 o Findings of the CPT investigation indicate that subsurface petroleum hydrocarbon impact extends to at least 60 feet bgs in the eastern portion of the site The extent of petroleum hydrocarbon impact to groundwater is not fully characterized o Petroleum hydrocarbon impact observed at well MW-7, and borings CPT-5 through CPT-7, likely has originated from the 7-11 facility 7.2 Discussion Relatively high concentrations of TBA, with associated MTBE, were reported in the depth discrete soil and groundwater samples collected at borings CPT-2 and CPT-3, which are situated in the eastern portion of the site to close proximity to the former eastern fuel dispenser island and K IDSA Gas01Lne10835acportslusa835-scm assessment report doc Page 15 STRATUS