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ARCADES GERAGHTY&MILLER as depicted for the Site by Applied Geosystems in May of 1990 Comparison of Figure 6-4 ' with 6-2 illustrates that the shallow benzene plume front has receded during the five year monitoring period Additional installation of monitoring wells at the Site have yielded ' information which has allowed unproved definition of the shape and orientation of the shallow benzene plume Given the direction of groundwater water flow through the period of the ' monitoring record it is not entirely clear that data from monitoring well U-7 is not influenced by the former presence of the gas station south across E Miner Avenue as shown on the 1972 ' Sanborn maps Nonetheless, the reduction in size of the shallow benzene plume over tune is evidenced by the declination in benzene concentration in wells U 1,U-2, U-4, and U-8 over the ' period over comparison ' The distribution of Site constituent concentrations in cross-section, drawn between well nests NP-1 and NP-2, is interpreted and illustrated in Figure 6-3 The mterpretation is based on a compilation of the Site geology using Site boring logs Constituent concentrations were overlaid on the cross-sectional Site geology Offsite hydrogeologic interpretation ' represent supple linear extensions of the stratigraphy observed in boring NP-1 and NP-2 Sample concentrations from the individual wells were plotted first and supplemented between screened intervals with SimulprobeTM groundwater grab sampling data The interpretation ' presented in Figure 6-3 reflects the field observation that preferential flow is occurring in the relatively higher permeability soil layers at depths approximately 60-65 ft bgs and at 1 approximately 110-120 ft bgs The 1,000 ug/L and 100 ug/L benzene contours intersect the piezometnc surface at locations consistent with the plan view interpretation shown in Figure 6- ' 2 The location of the 1,000 ug/L contour as shown in the mid-level water zone is interpreted using quarterly monitoring data collected February 5, 1999 Downgradient spacing of the log- linearly extrapolated 100 ug/L, 10 ug/L and 1 ug/L contours in the mid-level zone and the deep zone is based on the maximum spacing of the corresponding contours as plotted in plan view on Figure 6-2 Thus, Figure 6-3 does not represents a true cross-sectional depiction, but rather a conservatively-based cross-sectional conceptual model of flow and transport consistent with, 42