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Perhaps these peaks are due to the presence of a non-gasoline compound, such as a chlorinated hydrocarbon, several of these have been detected in VM-I and VM-2 in the past 4.3 Concentration Trends r Figure 4 shows hydrocarbon concentrations in VM-2 since monitoring began in July 1996 This graph shows a highly unusual trend both TPH-g and total BTEX concentrations have increased over time Although low concentrations have been recorded on several occasions, the overall trend is clear Presently, concentrations are approximately 1 order of magnitude greater than they were in 1996 A similar trend of increasing concentrations has not been observed in the other wells in VM-1, for example, concentrations remained relatively constant from 1995 through 1998, but then the TPH-g concentration spiked to its highest level in early 1999 (Figure 5) BTEX concentrations did not follow suit, and actually declined in early 1999 and remained low until early 2000 After peaking in early 1999, the TPH-g concentration began a long decline, and the present concentration appears to be slightly below the initial level in 1995 However, BTEX began rising as TPH-g was declining At the end of 2000, both TPH-g and BTEX concentrations were very low, and since then they have risen and fallen in phase This in-phase pattern is a more typical situation, and easier to interpret than the out-of-phase variations that were observed prior to 2001 Possible explanations for the observed patterns are discussed below 5.0 CONCLUSIONS 5.1 Hydraulic Gradient The hydraulic gradient returned to a more normal trend this quarter, and groundwater resumed its northeastward flow This confirms our conclusion in the Third Quarter 2001 Report that the southeast flow direction observed in September was a transient situation due to a lack of equilibration in the depth to groundwater in VM-4 5.2 interpretation of Concentration Trends As discussed in section 4 2, hydrocarbon concentration trends in the monitor wells are not following patterns that are typical of most sites Rather than remaining relatively constant or declining slowly in all wells, concentrations have remained constant, declined, or increased in various wells When only four wells are involved, this is an unusual situation At least four possible interpretations should be considered 5.2.1 Hypothesis: Northeastward Migration of the Contaminant Plume One possible explanation for the increase in hydrocarbon concentrations in VM-2 is migration of the dissolved-phase gasoline plume from the former UST area in the western portion of the site This interpretation is moderately compatible with the hydraulic gradient data, although gradient contours have implied that flow has usually been more to the east than to the northeast • 4 II