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clatog"I Te&da 1xc. Page 7 Site Conceptual Model Report Marci Property Project No 7012 August 23,2004 I Calculation of contaminant mass is difficult for many reasons • 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 1992 During the period of site characterization I 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 ' e 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 ' petroleum hydrocarbons will generally be adsorbed to soil particles Groundwater plume dimensions are defined by quarterly sampling of the monitoring wells at the site It is extremely difficult to characterize the mass of soil gas at a site and it is generally not done GTI has attempted to calculate the mass of TPH-D in both the soil and groundwater at the site The discussions of how the plume dimensions were modeled are included below Appropriate figures are included to aid in the demonstration of plume dimensions 2.5:1- Soil Plume T __ The TPH-D soil plume was modeled as two vertical cylinders, shallow (10-30') and deep (30,65'), see Figure 10 for plan view Figures l lA and 11B are cross sections showing ' plume dimensions ❑ The shallow zone is a cylinder that extends in a 19 foot radius from the midpoint between MW-4 and the center of the former UST excavation This area includes those samples from the 10-30 foot depth that had detectable contamination ❑ The deep zone cylinder extends from the bottom of the shallow zone at 30 feet bgs to the current water table at 65 feet bgs This area is centered on the former UST excavation and extends outward in a 40 foot radius 1