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1 <br /> AIRGADIS GERAGHTY&MILLER <br /> 6.4.3 Conceptual Model of Site Constituent Distribution and Migration <br /> -3 rcross-sectional view and cone tual model of the <br /> Figure 6 illustrates an interpreted p <br /> cross-sectional distribution of benzene between well nests NP-1 and NP-2 The cross sectional <br /> interpretation is based on use of the February 5, 1999 quarterly monitoring data and non- <br /> adjacent S Simulprob6m data collected on November 16-18, 1998 for NP-1, and January 8- <br /> cross section based on observations of the stratigraphy recorded on the boring logs of NP-1 <br />' and NP-2 The cross-sectional groundwater constituent distribution illustrated in Figure 6-3 is <br /> consistent with both the quarterly monitoring data, the Sunulproberm groundwater sampling <br />' data, and with the constituent distribution indicated by the Simulprob6m soil sampling data <br /> The conceptual model also considers the effects of the hydrogeology of benzene distribution <br />' The methods used to develop this conceptual model are explained in additional detail in Section <br /> 6 5 2 of this report <br /> Site hydrogeologic observations of the geologic profile derived from NP-1 and NP-2 <br /> 1 and based on continuous soil sampling and groundwater sampling on approximate 10-foot <br /> t intervals, indicate that two primary zones of relatively high pereabih�moccur beneath the Site <br /> The first zone was encountered at an approximate depth of 6l feet bgs_(referred to in this <br />' report as the mid-level zone) This is also the zone, at both the NP-1 and NP-2 sampling <br /> locations, in which the highest concentrations of TPHg and benzene were recorded The <br />' second relatively high permeability zone was encountered at approximately 116 feet <br /> ground surface (referred to in this report as the deep-Ievel zone) TPHg and benzene <br />' concentrations were significantly attenuated in the deep-level zone as compared to those <br /> detected in the nud-level zone Generally, concentrations of benzene decline with depth below <br /> the mid-level zone and then increase again, albeit to much lower levels, in the deep level zone <br />' It is interpreted that constituent migration is occurring primarily in the shallow ground <br /> water zone, and in the mid- and deep-level zone of relatively higher permeability The data <br />' 40 <br />