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Report of Findings Frontier Transportation 0200-024 March 3, 2000 Page b ' Total dissolved BTEX concentrations in MW-3 increased from 496 µg/L in November 1999 to 571 �ig/L in January 2000. However, total BTEX has attenuated 93% from the maximum 8,350 pg/L when groundwater rose to its shallowest depth recorded (7,1' bg) in March 1998. See Figure 8 for the recent benzene isoconcentration map. MTBE was identified in groundwater from three monitoring wells only. See Figure 9 for the recent MTBE isoconcentration map. Groundwater samples collected from monitoring wells MW-1, MW-3, and MW-9 identified MTBE concentrations at 170 pg/L, 90 µgfL, and 19 pg/L, trespectively, using EPA method 8260. MTBE concentrations have attenuated in MW-1 and MW-3 since November 1999 from 780 µgfl- and 540 pg/L, respectively. The detection of the highly soluble MTBE compound in groundwater collected from MW- 9, however, supports that MTBE may be migrating downgradient to the west beneath the site. No detectable concentrations of the remaining EPA method 8260 analytes were identified from groundwater sample analysis. 5.0) CONCLUSIONS Based on the site assessment results, the vertical extent of significant petroleum hydrocarbon impacted soil was defined at up to 15' bg near the former USTs in boring B-6. Soil sample analysis results from boring B-6 identified elevated levels of TPH-g, BTEX, and MTBE at 10' bg. However, levels of TPH-g, BTEX, and MTBE attenuate by 99% at 15' bg in boring B-6 from 100 mg/kg to <1 mg/kg, from 2,343 µg/kg to 34 pg/kg, and from 10,600 µg/kg to 37 µg/kg, respectively. ' It is concluded that the area of contamination is limited laterally in the subsurface soil since no other boring or well identified any detectable petroleum hydrocarbon concentrations except MW-8, which identified 10 µg/kg exclusively as benzene at 5' bg. It is further concluded that the vertical extent of petroleum hydrocarbon impacted soil near the former USTs is limited to 15' bg based upon collected soil sample results. It is also concluded that the lateral extent of groundwater contamination is limited laterally and vertically. Groundwater sample analysis results identified detectable concentrations of dissolved TPH-d, TPH-m, TPH-g, and BTEX in monitoring well MW-3 exclusively. The other ten monitoring wells, which surround MW-3 in all directions, identified no detectable concentrations of dissolved TPH-d, TPH-m, TPH-g, and BTEX. Analysis of groundwater from monitoring wells MW-8 through MW-1 l identified the lateral extent of dissolved TPH-d, TPH-m, TPH-g, and BTEX concentrations. Groundwater sample results support that dissolved