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7-Eleven Store#19976 Quarterly Groundwater Monitoring and Remediation Progress Report (1Q2006) May 16, 2006 • Page 3 micrograms per liter (pg/L) benzene and 290,000 pg/L TPHg Dissolved MtBE levels ranged up to 3,700 pg/L Groundwater monitoring wells MW-1/1A, MW-2, and MW-3 were installed in September 1998 (Figure 2) Well MW-1/1A is a dual-nested well completed to depths of 30 and 50 feet bgs Soil samples collected during well installation contained up to 0 07 mg/kg benzene and 0 08 mg/kg MtBE The highest concentrations were at 20 and 25 feet bgs (capillary fringe) in wells MW-1 and MW-2 Soil boring SB-1 and nested wells MW-4/4A and MW-515A were installed in May 1999, as requested by the SJCEHD, to provide additional lateral and vertical plume delineation Soil boring SB-1 was advanced to 66 feet bgs near the UST area, to evaluate the vertical extent of the plume The nested wells were installed to access the saturated zone at depths of 10- 30 feet bgs. (MW-4 and MW-5), and 45-50 feet bgs (MW-4A and MW-5A) Sod sample data Gas from boring SB-1 indicated that the base of the benzene, plume was at between 40 and 55 feet bgs, as evidenced by non-detectable concentrations of constituents Samples from well MW-5 contained low concentrations of gasoline "j-,— constituents from 30-35 feet bgs, with no detectable concentrations at deeper depths izc-tw (Shaw, 1999) Well MW-6 was installed in December 1999, to a depth of 30 feet bgs (Figure 2) Benzene and MtBE were not detected above laboratory reporting limits in soil samples The maximum TPHg concentration in soil was reported at 20 feet bgs (15 4 mg/kg) Delineation of hydrocarbons in soil was considered complete at this time (Shaw, 2000) On September 18, 2000, as requested by the SJCEHD, eleven soil-gas survey points were advanced to 3 feet bgs Significantly elevated MtBE concentrations (2,200 pg/L) were detected in only one of the points (SG-9), located along the western edge of the UST pit about 20 feet south of MW-2 On November 1, 2000, monitoring well MW-7 was installed to a depth of 30 feet bgs and soil boring SB-2 was advanced to a depth of 45 feet bgs (Figure 2) Sod samples analyzed from both well MW-7 and soil boring SB-2 did not contain benzene, toluene, ethylbenzene, xylene (BTEX), TPHg, and MtBE above laboratory reporting limits A grab groundwater sample was collected from boring SB-2 at a depth of 40 feet bgs ample ✓rjj co Iota - -5-pg/ _ Soil vapor extraction (SVE) testing c/rya, conducted in 2001 using the two horizontal vent wells installed in the former UST tank pit (IT Corporation, September 2001) indicated extracted vapor concentrations of over 10,000 parts per million by volume (ppmv) at applied vacuums ranging between 9 and 32 inches of water Induced vacuum responses of more than 0 2 inches of water were observed in wells MW-1, MW-2, and MW-3 during extraction from the horizontal wells Vacuum response in these wells was likely facilitated by relatively higher-permeability conditions in the tank cavity backfill as opposed to native sods Testing was also conducted on the groundwater monitoring wells, lower permeability and limited screen above the water table likely accounted for low extraction rates/low induced vacuum responses observed during extraction from the wells Extracted vapor concentrations ranged up to 0 91 ppmv 19976 06-1Q Report doc SECOR International incorporated