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a . ASS OCIATE S I N C . trace amount of TPHg and TPHd collected from the capillary fringe in MW1. 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 Usingthe analytical results from the subsurface investigations resented in Table 1 ATC Y g p ' -aajnLate —the lateral extent of TPHss, TPHd, 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 TPHss remaining in the soil is estimated to be approximately 1,302 pounds or 592 kilograms. The total mass of TPHd remaining in the soil is estimated to be approximately 1,384 pounds of 629 kilograms. The total mass of TPHg remaining in the soil is ' estimated to be approximately 524 pounds or 238 kilograms. The total mass of BTEX remaining in the soil is estimated to be approximately 4.64 pounds or 2.11 kilograms. Mass balance calculations are included in Appendix D. ' 4.2 Groundwater Conditions 4.2.1 Site Hydrogeology ATC performed quarterly monitoring of wells MW1, MW2, MW3, and MW4 in March/April, ' June, September, and December 2000. During this period, groundwater was measured at depths ranging from 22.22 to 25.41 feet bgs. The groundwater flow direction was consistently east- northeast to northeast at a gradient of 0.001 to 0.002 ft/ft. Groundwater contour maps are included in Appendix B and groundwater elevation measurements are presented in Table 3, 4.2.2 Petroleum Hydrocarbons in Groundwater Petroleum hydrocarbons have historically been detected only in well MW3 and generally decreased over the monitoring period with the exception of a trace amount of TPHd detected in wells MW1 and MW2 in December 1998 and March 1999. No petroleum hydrocarbons or fuel oxygenates were detected in any of the groundwater samples collected from wells MW1 and MW2 since March 1999 or from well MW4 since the well was installed in April 2000. 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 TPHss, TPHd, TPHg, and BTEX 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 MW3 has contained detectable concentrations with the exception of a w:\17295\rcports\dosure.doc 5