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:- Work Plan Frontier Transportation 1099-166 _ October 27, 1999 ' i from monitoring wells MW-5, MW-6, and MW-7 using EPA method 8020 since initially tested. _ Groundwater analyzed from wells MW- 1 , MW-2, MW-3 , and MW-4 identified attenuation in total BTEX from the maximum concentrations identified. Total BTEX concentrations in MW4 attenuated 99% from a maximum of 302. 6 pg/L in August 1996 to 2.08 pg/L in July 1999. Monitoring well MW-2 identified an attenuation of 99% of total BTEX concentrations from the jmaximum of 145 . 1 µg/L observed in August 1996 to non-detectable concentrations in July 1999. Total BTEX concentrations also attenuate significantly in MW-3 from a maximum of 8,350 µg/l, identified in March 1998 to 33 .2 gg/L in July 1999 . Monitoring well MW-4 likewise identified an attenuation of 99% of total BTEX concentrations from the maximum of 6 pg/L observed in September 1998 to non-detectable concentrations in July 1999. See Figure 5 for Benzene Isoconcentration Map. � J MTBE was identified in groundwater in four monitoring wells (MW- 1 , MW-2, MW-3, and MW4) only since initially tested. Monitoring wells MW-5, MW-6, and MW-7 have =� identified no detectable concentrations of MTBE since initially tested in March 1998. Maximum _ MTBE concentrations were identified as 1 ,300 gg/L and 12.3 gg/L from monitoring wells MW- 1 and MW-2 in August 1996. Since then, MTBE concentrations attenuated to 530 µg/L and non- detect, respectively, in July 1999. MTBE concentrations were identified at maximum levels in — monitoring wells MW-3 and MW4 as 29,400 pg/L (March 1998) and 42 ltg/L (December 1998), respectively. Concentrations of MTBE attenuated by 98% to only 360 µg/L from MW- 1 in July 1999. Attenuation of MTBE concentrations was also identified in MW4 to non-detectable levels in July 1999. See Figure 6 for MTBE Isoconcentration Map. 5.0) CONCLUSIONS AND RECOMMENDATIONS it _ Based on all data collected, BEII concludes that the vertical extent of soil contamination has not defined near the former USTs. Twinning advanced four borings to a maximum depth of 11 ' bg near the former USTs. Soil sample results identified 1 ,200 mg/kg as TPH-g at I F bg. No = soil samples were collected below 11 ' bg. In addition, dissolved petroleum hydrocarbons appeared to be localized near the former UST location and monitoring well MW-3 in shallow groundwater with the exception of MTBE. MTBE concentrations above regulatory allowable levels have been identified in monitoring well MW- 1 located approximately 110'west of MW-3 . The groundwater gradient beneath the site flows in a western direction. The elevated concentrations of the MTBE downgradient of the former USTs support that lateral migration of the plume may be occurring. 6.0) SCOPE OF WORK BEII recommends that a soil vapor survey be performed to determine whether vapor transport of petroleum hydrocarbons is a source of lateral migration. In addition, four (4) 2-inch groundwater wells (MW-8 through MW- 11) and one boring (13-5) will be advanced to 20' bg to delineate the vertical extent of soil contamination near the source area and the lateral extent of z II