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' 27 October 2004 • AGE-NC Project No 96-0254 Page 5 of 10 t5.0. GROUND WATER MONITORING PROGRAM Quarterly ground water monitoring has been ongoing at the site since January 2000 TPH-g and ' TPH-d have been detected only in two (MW-1 and MW-2)of the eight monitoring wells, and with the exception of 8 2 ug/1 xylenes detected in well MW-1 in August 2000, BTEX compounds have not been detected in ground water samples collected from the site since sampling began-MTBE has 1 been detected in all wells except MW-6 and MW-8, MTBE has been detected eac�quarter lin the samples collected from wells MW-1 and MW-2 at concentrations ranging from Z µg/1 to , µg/1, and rote ittently in wells MW-3 through MW-5 and MW-7 at concentrations ranging from 0 51 µg/1 to 210 µg/1 Based on the investigation to date ground water at the site is only impacted with MTBE at concentrations requiring active site remediation_'The vertical extent of MTBE-impacted ground4'? water has been defined at levels below:30 microg�r maYna s_per_liter~. The lateral extent of MTBE- impacted groundwater has generally been defined,to levels required for site assessment and closure tFurther assessment may be required to fully define the MTBE-impacted ground water vertically4Ac& beyond 50 feet bsg ' • MTBE concentrations in the most impacted wells MW-1 and MW-2 have declined noticeably since monitoring began in 1999 (Tables 4 and 5) A trend analysis for MTBE in well MW-1 showed exponentially declining concentrations over time at a correlation coefficient(accuracy)of 93(93%) ' (Appendix B) Utilizing a logarithmic presentation of the data(Appendix B), AGE estimated that since 2001,MTBE had attenuated at an approximate rate of 2,500µg/1/yr At the current decay rate, ' MTBE concentrations in well MW-1 will achieve the state secondary maximum contaminant level for drinking water of 5 µg11 by year 2008 (Appendix B) and the maximum contaminant level of 13 µg/1 sooner ' Analytical results of ground water samples are summarized in Tables 4 and 5 Cross sections depicting the general lithology in the soil beneath the site are illustrated on Figure 4 The lateral ' extent of dissolved MTBE is depicted on Figure 5 The trend analysis of dissolved MTBE in well samples is shown in Appendix B MASS OF DISSOLVED MTBE Based un giound watei analytical results for July 2004,AGE estimated the areal extent and mass of - dissolved MTBE in ground water (Figure 5) AGE assumes the following conditions ' The distribution of dissolved hydrocarbon concentrations on-site can be approximated by an ellipse area with a column depth to the deepest ground water sample collected, containing the higher MTBE concentrations, ' Advanced Geo Environmental,Inc