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,-(VSATC A S S O C I A T E S I N C detected in soil samples collected from nearby borings SP-2, SP-3, and SP-4 or from perimeter soil borings MW-2 and MW-3 with the exception of toluene, ethyl benzene, and xylenes detected at very low concentrations from the capillary fringe in MW-2. As a result, the lateral extent of residual petroleum-affected soils appears to be limited to the immediate vicinity of the site's former UST location as shown on Figure 5. 4.1.3 Mass Balance Calculations-Soil Using the analytical results from the subsurface investigations presented in Table 1, ATC estimated the lateral extent of TPHg and BTEX concentrations in the unsaturated zone beneath the site as depicted on Figure 5. The isoconcentration shown on Figure 5 was then used by ATC to perform mass balance calculations to estimate the mass of residual soil contamination. The total mass of TPHg remaining in the soil is estimated to be approximately 915 pounds or 415 kilograms. The total mass of BTEX remaining in the soil is estimated to be approximately 203 L, pounds or 92 kilograms. The total mass of MTBE remaining in the soil is estimated to be approximately 0.11 pounds or 0.05 kilograms. Mass balance calculations are included in Appendix D. 4.2 Groundwater Conditions -� 4.2.1 Site Hydrogeology ATC performed quarterly monitoring of wells MW 1, MW2, and MW3 in March, June, September, and December 1999. During this period, groundwater was measured at depths ranging from 33.96 to 37.14 feet bgs. The groundwater flow direction was consistently east- northeast to east at a gradient of 0.002 to 0.003 ft/ft. 4.2.2 Petroleum Hydrocarbons in Groundwater Petroleum hydrocarbons have historically been detected only in well MW 1 and generally decreased over the monitoring period with the exception of a trace amount of toluene detected in well MW2 in March 1999. No petroleum hydrocarbons or fuel oxygenates were detected in any of the groundwater samples collected from well MW3 or from well MW2 during the other three sampling events. ATC estimated the lateral extent of impacted groundwater as depicted on Figure 6. 4.2.3 Mass Balance Calculations-Groundwater Using groundwater analytical results presented in Table 2, the most recent data available, ATC estimated the lateral extent of dissolved TPHg, BTEX, and MTBE concentrations beneath the site as depicted on Figure 6. Figure 6 was then used by ATC to perform mass balance calculations to estimate the mass of residual contamination in the site's saturated zone. Historically, only well MW 1 has contained detectable concentrations with the exception of a trace amount of toluene w:W2574.05\repotU\closuraFS3.doc 4