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• Nevertheless, VM-2 is downgradient of VM-1 and eastward groundwater flow from VM-1 to VM-2 would be expected to transport dissolved hydrocarbons to the latter well Concentrations would thus increase in VM-2 while decreasing in VM-1 (as they have since 1998), but concentrations in VM-2 could not increase above those that were previously present in VM-1 However, as Table 2 and Figures 4 and 5 show, concentrations in VM-2 since 1999 far exceed any that have been detected in VM-I Hence, migration of the dissolved-phase plume could not be the only process involved 5.2.2 Hypothesis: Leaching of Hydrocarbons from Soil A second possibility is that adsorbed hydrocarbons in the vadose zone in the vicinity of VM-2 have been leached from the soil and carried downward to groundwater Gasoline was detected in samples at 15 and 20 feet from this boring when it was drilled in 1996, and BTEX compounds were detected as deep as 35 feet In addition, the area around VM-2 is the only location where hydrocarbon vapors (up to 970 ppmv) were detected during a soil-vapor survey that was performed in March 1998 Prior to 1998, the depth to groundwater exceeded 40 feet at the site, meaning that the gasoline in the 15-20 feet depth interval was separated from groundwater by at least 20 feet of vadose-zone soil, which could explain why hydrocarbon concentrations remained constant in the well through 1997 and 1998 (except for August 1998) By the middle of 1998, groundwater had begun rising rapidly, reaching 33 feet, and may have received a charge of gasoline if the gasoline had leached downward 10-15 feet during the preceding two years Leaching could also account for the sharp spike in the TPH-g concentration in VM-1 in early 1999 Groundwater 4t that time was only 8 feet below the high soil concentration (TPH-g =2,400ti mg/kg) that was detected at 25 feet when this well was drilled However, this hypothesis cannot explain why concentrations have continued to rise in VM-2 while declining in VM-1, nor does it explain why fluctuations in the TPH-g and BTEX concentrations in VM-1 were out of phase prior to 2001 5.2.3 Hypothesis: Leaching Followed by Eastward Plume Migration A combination of the previous two processes is better able to explain the high concentrations that have been detected in VM-2 If vadose-zone hydrocarbons near and west of VM-2 leached downward to groundwater and then migrated toward VM-2, concentrations could rise above those that have been observed in VM-1 Laboratory data from soil samples that were collected during the UST removal and from borings (including VM-1) that were drilled later offer some support to this hypothesis TPH-g concentrations ranged from 3 5 to 27 mg/kg in three of the four samples that were collected from beneath the UST's during their removal, and five of forty soil samples that were collected from the soil borings contained hydrocarbons, including one sample (SB-1-3 at a depth of 14 feet) that contained TPH (reported as Stoddard Solvent) at a concentration of 183 mg/kg Most BTEX compounds were also detected in these samples (up to 50 mg/kg xylene in one UST sample and 120 mg/kg xylene in one boring sample) Except for one boring, these samples were located south of VM-1, making in possible that subsequent northeastward or eastward migration of the contaminants would be detected in VM-2 but not in VM-1 r 5