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Quarterly Groundwater Report Frontier Transportation 0501-131 May 17, 2001 Page 4 have attenuated by 97% since the maximum was identified as 8,350 µg/L in March 1998 when groundwater rose to its shallowest depth recorded (7.1' bg). See Figure 5 for the recent dissolved BTEX Isoconcentration Map. MTBE was identified in groundwater from one monitoring well (MW-3) only. Groundwater collected from monitoring well MW-3 identified an increase in MTBE concentrations from 31 pg/L to 250 pg/L, since the previous monitoring episode (December 2000) using EPA method 8260. Groundwater in MW-3 rose 0.4' between the two episodes. The rise groundwater may have dissolved adsorbed MTBE in the soil. However, MTBE concentrations have attenuated by 99% from the maximum concentration of 29,400 µg/L identified in MW-3 during March 1998. No detectable concentrations of the remaining oxygenates were identified from groundwater sample analysis of all eleven monitoring wells using EPA method 8260 (See Appendix B). See Figure 6 for the recent MTBE Isoconcentration Map. 5.0) CONCLUSIONS AND RECOMMENDATIONS Based on groundwater sample analysis results, the dissolved TPH-d, TPH-g, TPH-m, BTEX, and MTBE plumes are spatially stable on-site and confined to the former UST location. No detectable concentrations of dissolved TPH-d, TPH-g, TPH-m, BTEX, and MTBE were identified in ten of the total eleven monitoring wells dedicated to the site. Dissolved TPH-g, BTEX, and MTBE were identified in monitoring well MW-3 exclusively located just east of former UST location. Attenuation of the above petroleum hydrocarbons has been identified in MW-3 since their respective recorded maximums. BEII recommends that quarterly groundwater monitoring continue at the site until constant or decreasing dissolved petroleum hydrocarbon concentrations are observed. Groundwater samples will be analyzed for BTEX using EPA method 8020, TPH-d and TPH-g using EPA method 8015m, and oxygenates using EPA method 8260 in all future monitoring 1 episodes as directed by the PHS/EHD. In addition, BEII requests that groundwater analysis for TPH-m will no longer be performed at the site.