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I ' 14 August 2000 . AGE-NC Project No 95-0128 ' Page 9 of 10 ' of MTBE detected in the deeper-screened wells MW-2, MW-3, MW-11B, and MW-12B were 10µg/l, 9 4,ug/l, 600,ug/l and 220µg11, respectively Compared to the 79,ug/l to 2,500 /.cg/1 MTBE detected in the properly screened wells in the core area and in the down-gradient ' area, the deeper concentrations are suggestive of minor to moderate vertical migration of MTBE, assuming that the MTBE is shallow-sourced • The MTBE concentration in the up-gradient wells, MW-6 and MW-7, was 130 µg11, and ' 140,ug/l,respectively,which was slightly greater than the concentrations detected in samples from the deeper-screened wells MW-2 and MW-3, also suggesting preferential horizontal ' migration of MTBE compared to vertical migration • The horizontal extent of TPH-impacted ground water is fairly well delineated, however, the horizontal extent pf MTBE-impacted ground water is not, especially toward the north The ' current ground water monitoring well system appears to he situated near the MTBE-impacted ground water plume margins on the east, south and west Figure 7 depicts the extent of dissolved MTBE above 500 µg/L ' • The narrow northward extension of more intensely MTBE-impacted shallow ground water that includes MWS,MW8, MW11 and MW12 is suggestive of a detached plume migrating ' • northeastward from the site, or of a plume extending from an active release, possibly from the northern or central dispenser island areas I 50 RECOMMENDATIONS IBased on the results of this and previous investigations, AGE recommends • Continued quarterly monitoring of the ground water monitoring network, • Immediate initiation of ground water extraction from MW5 to prevent further of-site ' migration of MTBE-impacted ground water, • Performance of a passive soil gas survey designed to identify unrecognized impacted soil and potential `slow' release point(s) that may not be revealed through normal tightness tests Pe11VI1lldilLe of d VdLlUI G1'CLidI�LIUIl test in L1lG J1ldlivW, 1111E rldlll JU11 LV LChL 1C Lb1U111Ly 1171 addressing impacted soil in the vadose zone The previous vapor extraction pilot test was conducted at a time of low ground water level that left a significant sand interval in the vadose zone, the results are not likely to be applicable to current site conditions Advanced CeoEnvironmental Inc