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Remedial Action Plan Frontier Transportation June 17, 2004 Page 4 1 encountered predominantly clays with traces of interbedded sands from 0' to 25' bgs during well advancement. See Figures 3 and 4. for lithologic cross sections. ¢a First subsurface water was encountered at depths from 11 ' to bgs during tank B I removal and monitoring well installation at this site in 1995 and 1996. Since then, first subsurface water has been encountered beneath the site between 6' and 10' bgs. Data collected in March of 2004 identified the flow of groundwater in a generally western rn-{ direction beneath the site (See Figure 5, Groundwater Gradient Map). Historic first subsurface groundwater data identified a west or soutwest flow beneath the site. Groundwater was also encountered at approximately ' bgs during installation of deep monitoring wells MW- 12 through MW- 16 and MW- 18 . The potentiometric groundwater levels in these deep wells, however, were similar to the first subsurface water levels. Groundwater levels beneath the site appear to be affected by seasonal recharge. In general, the groundwater is at its yearly low in the winter and yearly high in the spring. 4.0) IMPACTED SOIL Soil sample results from the 1995 UST removal identified petroleum hydrocarbons in the northeast excavation pit where gasoline was stored. Four soil samples were collected from the pit bottom (8 . 5' bgs) and identified TPH-g between 9,800 mg/Kg to 4,700 mg/Kg and BTEX between 1 ,007 mg/Kg to 1 ,354 mg/Kg (See Appendix A for soil sample results). Laboratory analysis indicates that the gasoline USTs were the source of impacted soil in the excavation pit bottom. The site assessment performed in 1996 focused on delineation of the source, the former gasoline USTs . Borings B- 1 through B-4 and monitoring wells MW- 1 through MW-3 were advanced to various depths up to 25' bgs. Laboratory analysis identified low level concentrations of TPH-g (<34 mg/Kg) and BTEX (<1 .365 mg/Kg) in those soil samples collected from shallow depths (6' to T bgs). MTBE was identified at a maximum of 30.5 mg/Kg at 6' bgs in boring B-2 located Wof the former USTs. Soil samples collected from 9 .5' to 11 ' bgs, the approximate depth to groundwater, identified i relatively high concentrations of TPH-g, BTEX, and MTBE. Concentrations of TPH-g were observed from 1 ,300 mg/Kg to 104 mg/Kg, BTEX from 259 .2 mg/Kg to 5 .23 mg/Kg, and MTBE from 110 mg/Kg to <0. 625 mg/Kg (See Appendix A for soil sample results). Laboratory analysis indicates that most of the shallow impacted soil identified by this investigation has been removed during UST excavation. Borings B- 1 through B-4 were located 2 feet to 18 feet away from the UST removal pit and identified low level petroleum hydrocarbons from 6' to T bgs . Moreover, the elevated TPH-g, BTEX, and MTBE concentrations from the capillary fringe and saturated zone (9 . 5 ' to 11 ' bgs) indicate that petroleum hydrocarbons have migrated through the soil and impacted the shallow groundwater. The contact between impacted groundwater and soil is the source of the lateral extent of TPH-g, BTEX, and MTBE identified in soil collected from B- I , 13- 21 B-3 , 134, MW- IRA, MW- IRB, MW-2RA, MW-2, and MW-3 at 9 .5' to 11 ' bgs . ( i