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iQuarterly Groundwater Monitoring Report Frontier Transportation 0899-139 August 13, 1999 method 8015 modified. Groundwater collected from monitoring wells MW-1 and MW-3 identified TPH-g concentrations as 75 pg/L and 100 gg/L, respectively. TPH-g concentrations increased slightly in MW-1 from 50 µg/L identified in February 1999. An attenuation of TPH-g concentrations was observed in MW-3 from 100 pg/L identified in February 1999. Please see Figure 6, for recent TPH-g isoconcentration map. No detectable concentrations of BTEX were identified in groundwater samples collected from monitoring wells MW-2, MW-4, MW-5, MW-6, and MW-7 using EPA method 8020. Groundwater analyzed from wells MW-1 and MW-3 identified total BTEX as 2.08 pg/L and 33.2 pg/L, respectively. Total BTEX concentrations in MWA attenuated 39% from 3.4 gg/L in February 1999 and 99% from 302.6 pg/L when initially tested in August 1996. Monitoring well MW-1 contained benzene at 0.58 pg/L and ethylbenzene at 1.5 pg/L exclusively as the total BTEX concentration identified in July 1999- Total BTEX concentrations in MW-I attenuated 94% from 588 pg/L in February 1999 and 99% from 8,350 pg/L,when groundwater rose to its shallowest depth recorded(7.1' bg) in March 1998. Monitoring well MW-3 contained benzene at 26 pg/L, ethylbenzene at 0.9 pg/L, toluene at 3.1 gg/L, and xylene at 3.2 gg/L as the total BTEX concentration identified in July 1999 when groundwater receded back to 10' bg. See • Figure 7 for Benzene Isoconcentration Map. MTBE was identified in groundwater from monitoring wells MW-1 and MW-3 only. Groundwater samples collected from monitoring well MWA located downgradient of the former USTs identified an increase in MTBE concentrations from 14 pg/L to 530 pg/L, since the previous monitoring episode(February 1999)using EPA method 8260. MTBE concentrations attenuate by 67% in MW-3 from 1,100 pg/L to 360 pg/L since February 1999. All other monitoring wells contained no detectable concentrations of MTBE. MTBE is highly soluble in water and will lead the dissolved plume. The increase in MTBE in MWA located downgradient from the former USTs supports that MTBE is migrating to the west beneath the site. 5.0) CONCLUSIONS AND RECOMMENDATIONS Based on groundwater sample analysis results, the dissolved TPH-g and BTEX appeared to be localized near the former UST location and monitoring well MW-3 in shallow groundwater. Groundwater collected from monitoring wells MW-1, MW-2, MW4, MW-5, MW-6, and MW-7 identified low level to no detectable concentrations of petroleum dissolved hydrocarbons (TPH-d, TPH-g, and BTEX). Concentrations support that the dissolved phase plume is confined to the site and is defined to the east, west, north and south of the former UST area. Groundwater collected from monitoring well MW-3 identified an attenuation in dissolved petroleum hydrocarbon concentrations since the previous monitoring episode. However, the - � 3