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i Ms Margaret Lagono ' Page 4 of 5 ' Top Filling Station Plume This plume is located immediately north of the ARCO plume and accounts for the contaminants detected in ARCO well MW4 and in Knowles wells MW3, MW4, MW7, MW 11, MW 12 and MW 14 Unlike the ARCO plume to the south and the ' Knowles plume to the north, this plume contains high concentrations of TPHg, BTEX and MTBE The cross-gradient limits of this plume are defined by ARCO well MW 1 to the south ' and by Knowles wells MW2 and MW7 to the north This plume is mobile and results from a relatively recent release as evidenced by the presence of MTBE and the rapidly increasing concentrations first observed in Knowles well MW4 (1997), then in Knowles MW3 (1998) ' and more recently in MW7 and MW 12 This plume presumably originates across Wilson Way at the Top Filling Station, 101 S Wilson Way ' Knowles Plume This plume is located north of the Top Filling Plume and is centered about the former USTs which were removed in 1978 The cross-gradient limits of this plume are defined by MW2 on the south and by MW9 and MW 10 on the north The Knowles plume is ' characterized by high concentrations of TPHg and BTEX without MTBE, as would be expected given the age of the release These plumes are illustrated on Figures 3, 4 and 5 which depict the isoconcentrations of MTBE, benzene and TPHg, respectively VAPOR EXTRACTION SYSTEM ACTIVITIES ' VES Operation The soil vapor extraction system was not operated during the third quarter of 2001 pending ' direction from PHSIEHD regarding further operation of the system Confirmation borings sampled during the first quarter indicate that no significant vadose soil contamination remains t RECOMMENDATIONS ' VES Operation ' Ground Zero recommends removal of the thermox system, and destruction of selected vapor extraction wells as discussed in our May 18, 2001 report ' Quarterly Groundwater Monitoring and Sampling Groundwater monitoring and sampling for the fourth quarter will be conducted in November 2001 PHS/EHD will be notified 48 hours prior to conducting the quarterly fieldwork ' _klq-\3Qv12001 i