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geologtcaI Tuhaks lac Page 12 Revised Site Conceptual Model Report Project No.507.2 j April 3,2008 r 1 _ t ...................... e MW-6 and MW-8. This-groundwater data�-supports the-conclusion^that te gasoline release i occurred in the vicinity of the alley between the_two.buildings,— Figures 12, 13 and 14 illustrate the relationship between the TPH-G concentrations and groundwater elevations over time in wells MW-9, MW-5 and MW-6 respectively. MW-9 (located in the core of the plume) data shows there is no clear trend but rather a variable relationship between these parameters. The linear trend line added to the TPH-G data in Figure 12 indicates that as the groundwater elevation has fluctuated, the concentration of TPH-G has remained stable. The polynomial trend line added to the groundwater elevation data illustrates the reversal in a declining trend over two consecutive winter/spring heavy recharge periods of the hydrologic cycle. S �} Trend lines added to MW-5 (located up/cross gradient of the core of the plume) data in Figure 13 show that as the groundwater elevation fluctuated since 1999 the concentrations of . TPH-G have decreased slightly. A similar relationship between groundwater elevation and contaminant concentrations exists in down gradient well MW-6. Trend lines added to the data in Figure 14 show that as the groundwater elevation has fluctuated since 1999, the concentrations of TPH-G in the well ? have decreased slightly. 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. 1 k-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 L and many low concentration samples around the edges of the plume. This is compounded by the spatial variability noted above. j • Extended period of time overwhich samples are collected. The first borehole soil and Lgroundwater samples were obtained in 1996 (MW-8 &MW-9). 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 n attenuation processes in the subsurface, such as biological activity. Due to this factor, J