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' ARC ADiS GERAGHTY&MILLER An examination of the historical groundwater monitoring analytic data in Table 1 of Appendix D shows that there are periodic fluctuations in the benzene data through the period of recorded monitoring, especially in wells located on the easterly and westerly margins of the plume as depicted in Figure 6-2 Recognizing that advective flow of a dissolved benzene plume is the primary mode of plume migration at the Site, the Figure 6-2 plot of benzene ' concentrations would indicate that the direction of groundwater would be to the northeast Yet,Figure 1 of the Appendix D quarterly groundwater monitoring report clearly indicates that ' flow is to the easterly direction The direction of groundwater flow appears to vary at the Site from approximately easterly to approximately northeasterly The periodic variation m ' groundwater flow direction is the likely mechanism to explain the periodic variation in benzene concentration as seen in the analytic monitoring record for Site wells It can be reasonably ' expected that similar variations in benzene concentrations would occur in the mid-level and deep-level zones beneath the Site The downgradient increase in concentration at well NP-2- 116 may not represent a significant increase above the concentration in NP-1-111, but rather represent fluctuation in the concentration values explainable by changes in the groundwater ' flow direction ' 6.4 VERTICAL EXTENT OF SITE PETROLEUM HYDROCARBON IMPACTS 6.4.1 Soil Impacts ' At the direction of SJCPHS, boring NP-1 was located less than 10 feet from the shallow monitoring well (U-3) with the current highest benzene concentration While this ' location posed the risk that the drilling would create a vertical contaminant pathway, it also provided a worse-case sampling point at which to delineate the vertical extent of impacts at the ' Site Table 5-1 and Figure 5-2 summarize the vertical delineation of the soil impacts beneath the Site as indicated by SnmulprobeTM soil sampling results from boring NP-1 Detections of TPHg and benzene in samples taken from the saturated zone in boring NP-1 occurred at depths ' 37