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G¢olagical Tecknics kc. Page 11 Site Conceptual Model Project No. 989.2 December 17, 2003 4.6.2 TPH-G Mass Estimate Calculations ' Figure 6 shows a contour map of TPH-G soil contamination as interpreted by soil collected during various drilling events. Figures 8 and 9 are cross sections showing the estimated boundaries of TPH-G contamination, in the subsurface, used to calculate the mass of ' contamination in the soil. Figure 7 is a contour map of TPH-G groundwater contamination as interpreted by analytical data from quarterly groundwater monitoring events. ' The mass estimate calculation, quantifying the amount TPH-G remaining at the site, is included as Table 6. Approximately 74.9 gallons of gasoline remain in the soil and approximately 0.1 gallons remain in groundwater at the site. ' 4.6.3 MtBE Mass Estimate Calculations Figure 10 shows a contour map of MtBE soil contamination as interpreted by soil collected during various drilling events. The contamination appears to cover an area of approximately 36,500 ft2. Figure 11 is a contour snap of MtBE impacted groundwater as interpreted by analytical data from quarterly groundwater monitoring events. The preliminary mass balance calculation, quantifying the amount MtBE remaining at the site, is included as Table 7. Approximately 0.078 gallons of MtBE remain in the soil and approximately 0.011 gallons remain in groundwater at the site. ' 5.0 POTENTIAL EXPOSURE PATHWAYS Utility trenches seem to be the most likely candidate for potential exposure pathways. A storm drain system runs along side monitoring wells MW-3 through MVV-6. The storm drain appears to move storm water run-off from Fremont Street, south of the site, along the alley ' between Larson Marine and the site, joining an east-west system along West Oak Street, north of the subject site. The storm drain system and other utilities buried on and off-site, may act as a preferential pathway for the migration of contaminants. Nevertheless, the plume ' appears to be stable and further migration seems improbable. ' The parking area of Valley Wholesale Drug Company and the surrounding sites and streets are capped in asphalt. A few small planter areas could facilitate infiltration. However, the planters are filled with vegetation and the surface area of these planters is minor. ' There are a few buildings off-site and downgradient of the plume. Factors that would make vapor migration an unlikely threat to off-site structures: 0 The plume appears to be stable Most of the area adjoining and adjacent to the source area is encapsulated with concrete i or asphalt 1