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i� 11 � d In 2002, MTBE was identified at 26,000 ppb in the soil at the heast corner of the building at 14' bgs, beneath the 10-foot deep groundwater table. MTBE and gasoline aromatics were identified at similar levels aroximately 40' northwest of the excavation in MW-13 at 10' bgs and 15 ' bgs: h of rW2 T j N!A i117 � = � 5fv0ufl � Sz = 11 crvo.Wy� �1� In March 2004, MTBE was identified dissolved in the groundwater at 100 ppb in monitoring well MW- 1 approximately 50' est-northwest of the mechanics shed. MTBE was identified at the same concentration, 1 „100 ppb, in monitoring performed in June y� 1tl 2004. MTBE concentrations increased to 8, 100 ppb in MW- 1 in monitoring performed I n September 2004. MTBE was identified in monitoring well MW-17 in all three monitoring episodes in 2004 at low and fluctuating concentrations, possibly defining the southern dispersion of the dissolved plume. MTBE has been identified at trace and fluctuating concentrations in five monitoring episodes in MW-9 since January 2000, possibly the western boundary of the dissolved plume. Neither significant MTBE nor other gasoline constituents were identified in the deep wells on site, which are all screened at 30' to 40' bgs, supporting the lack of vertical leakage through the soil beneath 20' bgs. The approved deep well south of MW-3R can confirm that no vertical leakage is taking place. Clean Soils, Inc. (CSI) proposes that two additional shallow wells be drilled. (Please see the attached map �g Fell locations.) One shallow screened well is proposed immediately west of the north s le of the mechanics shed to address migration under the northern portion of the building. Significant migration of the discharged gasoline under the northeast comer of the mechanics shed appears to have occurred based upon the high levels identified in the soil immediately �ove the first groundwater and capillary fringe , in monitoring wells MW4 and MW- 13 through MW48. In addition, the consistent and presently increasing MTBE concentration in MW- 1 indicates that westward migration may be ongoing. Soil analysis using EPA Methods 8015 and 8260 will provide information to further assess contaminant migration. CSI also proposes that one additional shallow screened monitoring well be placed h? �d northwest of MW4 where dissolved MTBE has been identified above 1,000 ppb for past three quarters with a significant increase in the last monitoring episode: This Te � u�/L shallow screened groundwater monitoring well be northeast ofMW4 5 where MTBE AU O 9G : 300 identified at4806 ppb immediately above the water table in the three 6deep screened o &t- 0 z 8o e wells MW- 14 t ough MW46 in that area. M w ' i 4 r, U1-c; jr I I') r g y I 'N 1131b ,Lryylpe `'"�." • l I , 310tit9 � ib �' . IL of 1s ' = 67 ) Our staff believes that the MTBE identified in the capillary fringe of the first groundwater in these wells has not migrated to the deeper screened wells because the aquifers are separated by relatively impermeable soil. If this conclusion is correct, th remediation can be limited to a maximum depth of 15 ' bgs to 20' bgs in that area.