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M Remedial Action Plan Frontier Transportation June 17, 2004 Page 5 r An additional site assessment was performed in 1998 to delineate the lateral extent of petroleum hydrocarbons identified from the above nine mentioned borings at 9 .5' to 11 ' bgs. Monitoring wells MW-4, MW-5, MW-6, and MW-7 identified no detectable concentrations of TPH-g, BTEX, or MTBE in all soil samples collected from T to 25 ' bgs and analyzed. See Appendix A for soil sample results. The soil sample results indicate that the lateral extent of impacted soil from 6' to T bgs was limited to borings B- 1 through B-4 only. Furthermore, the lateral extent of impacted soil from 9 .5 ' to 11 ' bgs via in contact with impacted groundwater does not extend to MW-4, MW-5, MW-6, and MW-7. Another site assessment was performed in 2000 to further delineate the lateral extent of petroleum hydrocarbons identified from 9. 5' to 11 ' bgs. Two borings (B-5 and B-6) and four monitoring wells (MW-8 through MW- 11) were advanced from 20' to 30' bgs . Only boring B-6 located within the former gasoline UST excavation pit identified petroleum hydrocarbons. Concentrations of TPH-g, BTEX, and MTBE were first detected in soil collected from B-6 at 10 bgs, the depth of groundwater beneath the site (See Appendix A for soil sample results). Trace levels of BTEX and/or MTBE were detected in soil from 15' to 30' bgs in boring B-6. No other soil samples analyzed from B-55 MW-8, MW-9, MW- 10, and MW- 11 identified any detectable concentrations of TPH-g, BTEX, and MTBE. The soil sample results indicate that the lateral extent of -_ shallow impacted soil from 6' to T bgs was limited to borings B- 1 through B-4. In addition, the lateral extent of TPH-g, BTEX, and MTBE in soil collected from 9 .5 ' to 11 ' bgs was limited to B- 1 , B-2, B-3 , B-4, B-6, MW- lRA, MW- 1RB, MW-2RA, MW-2, and - MW-3 . A supplementary site assessment was performed in 2002 to remove additional impacted soil from the former gasoline UST farm location and quantify the vertical extent of petroleum hydrocarbons trapped under the shallow first water. A pit 30 feet by 30 feet and ranging 13-feet to 15-feet deep was excavated at the former gasoline UST farm location. Soil samples were collected at 4' and 12' bgs from the excavation pit. f j During excavation, major water seepage occurred in the excavation pit at 11 ' bgs. Soil samples were also collected below water seepage and ponding at 14' bgs. Laboratory analysis identified low level petroleum hydrocarbons at 4' bgs increasing 1 ,000 times or 3 orders of magnitude greater concentrations of TPH-g, BTEX and MTBE below the first water at 14' bgs (See Appendix A for soil sample results). Approximately 325 tons of impacted soil was removed for off-site disposal. l Also in 2002, three monitoring wells (MW-3R, MW- 12, and MW- 13) were advanced from 20' to 40' bgs. Monitoring well MW-3R was installed to replace MW-3 , which was abandoned prior to the excavation. Monitoring wells MW- 12 and MW43 were advanced to 40' bgs to determine the lateral extent of petroleum hydrocarbons from -_ boring B-6. Soil sample analysis identified low-level petroleum hydrocarbons at 5' bgs in MW- 13 . Elevated concentrations of TPH-g, BTEX, and MTBE were identified in soil - I from MW- 13 at 10' bgs. Trace levels of BTEX and/or MTBE were detected in soil from