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Rantec December 8,2010 Page 2 of 9 mg/kg benzene, 2,600 mg/kg TPHg, and 27 mg/kg MtBE. The highest concentrations of hydrocarbons in soil were detected in boring GP-3 at a depth of 35 feet bgs. Grab groundwater samples collected from the base of each boring contained up to 5,800 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-5/5A 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 �a5 the plume. The nested wells were installed to access the saturated zone at depths of 10 to 30 04T feet bgs (MW-4 and MW-5), and 45 to 50 feet bgs (MW-4A and MW-5A). Soil sample data from et boring SB-1 indicated that the base of the benzene, TPHg, and MtBE/oxygenate plume was b9� between 40 and 55 feet bgs, as evidenced by non-detectable concentrations of constituents. Samples Samples from well MW-5 contained low concentrations of gasoline constituents from 30 to 35 feet bgs, with no detectable concentrations at deeper depths (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, 11 soil-gas survey points were advanced to 3 feet bgs (Figure 2). Significantly elevated MtBE concentrations (2,200 pg/L) were 4— detected in only one of the points (SG-9), located along the western edge of the UST pit approximately 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. Soil samples analyzed from both well MW-7 and soil boring SB-2 did not contain 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. The L. Soil vapor extraction E— (SVE) testing 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 MWA, 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 soils. 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 benzene, 220 ppmv MtBE, and 880 ppmv TPHg. The data indicated that vapor extraction was a viable remediation method at the site, particularly when the horizontal wells were used. 19976 Vapor Risk Assessment Report.doc