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I I20 October 2001 AGE-NC Project No 95-0111 ' Page 7of8 three most recent monitoring events, defining the north and southwest boundaries of the on- site MTBE plume • MTBE concentrations have increased significantly in off-site wells MW-5, MW-6 and ' MW-8 Based on information provided by the EHD, an unauthorized release of petroleuin hydrocarbons occurred at a property located approximately 250 feet east of the site and is likely the source for the recent increase in MTBE concentrations ' • The fuel oxygenate tertiary-butal alcohol (TBA)was detected intermittently in wells MW-4 and MW-5 between 1998 and 1999,TBA has not been detected in the eight monitoring wells ' since December 1999 Analytical results of ground water samples are summarized in Tables 4 and 5 The lateral extent of ' hydrocarbon-impacted ground water based on the most recent analytical data is illustrated in Figure 8 Time versus concentration graphs of TPH-g,TPH-d,benzene and MTBE for wells MW-1 through MW-8 are Included in Appendix E ' • 6.0. DISCUSSION AND CONCLUSIONS Based on data collected during site assessment and ground watermonitoring events,AGE concludes ' c Depths to ground water from wells MW-1 through MW-8 were measured between 5 97 and 1147 feet below the top of well casings Ground water at the site has consistently had a ' northerly flow direction under an average gradient of approximately 0 008 foot/foot O Concentrations of TPH-d,TPH-g and BTEX constituents have gradually decreased at the site to non-detect levels in wells MW-1, MW-2, MW-3, MW-6, MW-7 and MW-8 However, ' relatively low concentrations of TPH-d were detected in the most recent monitoring event conducted in June 2003 The dissolved hydrocarbon plume appears to be well-defined and stable Tables 4 and 5 summarize analytical results for all ground water samples collected at the site a MTBE concentrations in the on-site ground water monitoring wells have remained relatively ' stable during the investigation However, MTBE concentrations in off-site wells to the east and northeast have increased significantly over the past year of monitoring According to information provided to AGE by the EHD, an unauthorized release of petroleum ' hydrocarbons occurred at a neighboring property approximately 250 feet west of the subject site High concentrations of MTBE were detected in ground water samples collected from the neighboring property during tank removal activities The source for the increased MTBE ' concentrations in the off-site wells is likely to be the unauthorized release at the neighboring property ' Advanced GeoEnvironmental Inc