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Gealogccalruknusi�. Page i 1 Site Conceptual Model Report Project No 724 2 May 2,2006 ' diagrams there aren't enough detection points so GTI did not attempt to draw a plume or perform calculations for soil contaminant mass ' 2.5.2 Groundwater Figures 10 and 11 illustrate the January 2006 CPT data and the February 24, 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 ' For the 1St Quarter 2006 sampling event none of the shallow wells contained TPH-G above the laboratory reporting limits The TPH-G distribution in the intermediate wells is illustrated in Figure 12A The TPH-G distribution in the deep wells is illustrated in Figure 12B 2.6 Contaminant Mass Estimate Calculations The total mass of gasoline petroleum hydrocarbons released at the site is unknown However, 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 o 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 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 1992 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 mass may continually decrease at a site while characterization progresses outward With sites where samples are collected over a long period of time, it is generally not possible to estimate how much concentrations have decreased in areas were samples were collected early in the investigation The contaminant plumes at the site consist of three phases adsorbed to the soil particles, dissolved in the groundwater and as vapor in the pore spaces of the soil Of these, the bulk of