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'0,41cal rechala Inc Page 11 Site Conceptual Model Report(2006 Revision) Valentine(Castle Motors) Project No 723 2 June 16,2006 Figures 12, 13 and 14 illustrate the April 17, 2006 groundwater TPH-G plumes for the shallow, intermediate and deep aquifer horizons, respectively GTI has presented an interpretation of the groundwater plume that deviates from the customary concentric circles or ellipsoids to more accurately define the groundwater plume in the shallow and intermediate aquifer horizons Due to less available data points the customary method was used in depicting the deeper aquifer as presented in Figure 14 ' Figures 15 and 16 illustrate the February 2006 CPT data and the April 17, 2006 monitoring event groundwater contaminant distribution as superimposed on cross sections A - A' and B — B' The groundwater data are presented in the diagrams adjacent to the respective well's ' screened interval or CPT sampling depth The aquifer horizons do not exhibit the same lateral extent for the edges of the plume and GTI believes this reflects the geologic heterogeneity of the subsurface In Figure 4 the sandy units at 60— 75 feet bgs in the vicinity ' of well MW-201 are probably acting as a preferential pathway for contaminant migration The silty/clayey layers above this depth would act to restrict the lateral migration of the contaminant plume The plume depicted in Figure 16 in the deeper aquifer level therefore ' extends more to the east than the intermediate aquifer The shallow aquifer table also has a greater lateral extent than the intermediate aquifer probably due to dispersion caused by the rise and fall of the groundwater table 2.6 Contaminant Mass Estimate Calculations ' The total mass of gasoline petroleum hydrocarbons released at the site is unknown. In order to determine the fate and transport of the contamination and hence the future risk these ' compounds may pose to human health, an estimate of contaminant mass is necessary Calculation of contaminant mass is difficult for many reasons ' 0 Spatial variability of contaminant concentrations, both laterally and vertical This variability is controlled by geology, soil moisture, contaminant type, etc Due to these variabilities, when contaminant concentrations are averaged between sample locations, the estimate may be either higher or lower than what is actually present • Insufficient data points Because site characterization activities usually focus on defining the extent of the plume, few borings, and hence samples, are collected from the central ' portions of the plume This generally creates a data set with a few very "hot" samples and many low concentration samples around the edges of the plume This is compounded by the spatial variability noted above ' + Extended period of time over which samples are collected The first borehole soil and groundwater samples were obtained in 1993-1994 During the period of site characterization activities, contaminant concentrations change for a variety of reasons, including but not limited to migration, degradation, dilution, and/or new sources adding to the existing levels • Difficulty in quantifying natural attenuation Contaminant mass degrades due to natural processes in the subsurface, such as biological activity Due to this factor, contaminant