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Ms. Margaret Lagorio Page 2 of 8 Installation of Additional Groundwater Monitoring Wells PHS/EHD in the letter dated July 20 2001 requested the installation of additional multi-level groundwater monitoring well(s) to further define the extent of contamination. We pKopose to install one multi-level monitoring point downgradient of the leading edge of the "Knowles Plume" of groundwater contamination. Proposed monitoring wells MW15S/MW15D will be completed in one borehole and will be screened in two zones. Well MW15S will be screened across the water table with a screened interval from approximately 35 to 55 feet below grade. Well MW15D will be screened from approximately 70 to 90 feet below grade, which corresponds to the zone of the highest soil and groundwater contaminant levels identified in the recent deep soil borings. The proposed location of the new wells is intended to complete the lateral definition of the plume of dissolved groundwater contamination that originates from the release at 102 S. Wilson Way. Based upon groundwater flow direction, subsurface lithology, plume geometry, and chemical composition there appear to be no fewer than 3 distinct plumes of groundwater contamination on the east side of Wilson Way, between Washington Street on the south and Market Street on the north: ARCO Plume: The most southerly is a plume of dissolved MTBE located east of the underground storage tanks located at ARCO Station 5469, 130 S. Wilson Way. The highest concentrations are present in wells MW5 and MW6. Wells MWl, MW2 and MW3 define the cross-gradient limits of the plume. The plume is notable for the nearly complete lack of TPHg and BTEX compounds despite having high concentrations of MTBE. Ton Filling 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, MW11, MW12 and MW14. 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 well MW3 (1998) and more recently in wells 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. Figures 3, 4 and 5 depict isoconcentrations of dissolved MTBE, benzene and TPHg, respectively, along with the most recent potentiometric surface elevation. As these figures illustrate, the roek\clo ekworkplm\respa e.doc