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i ' Mr Nuel Henderson . Page 8 of 9 ' 1,2-DCA was detected in wells MW2, MW12, MW15A and MW15B Analytical results for gasoline oxygenates, EDB and 1,2-DCA are summarized in Table 5 Attachment E contains laboratory ' reports and chain of custody documentation Vapor Extraction Wells ' Petroleum hydrocarbon concentrations in the sampled vapor wells were within historical ranges EDB was detected in the sample from vapor well VW 1 TPHg and BTEX concentrations are ' summarized in Table 4 1,2-DCA, EDB and gasoline oxygenate additive concentrations are summarized in Table 5 Attachment E contains laboratory reports and chain of custody documentation CONCLUSIONS ' + Groundwater elevation data continue to show an east-northeasterly groundwater flow direction Water level data from the well cluster MW 15A, MW 1513, MW 15C, located furthest ' downgradient from the release, indicate an upward vertical gradient component at that point The vertical gradient component at the well cluster MW6AIMW I6 was downward ' + The analytical data from wells MW 15A, B and C provide downgradient definition of the extent of dissolved contamination The trace levels of BTEX compounds detected in wells MW 15B and MW15C following development and initial sampling were not detected during the subsequent round of sampling conducted in February These wells appear to be at the downgradient limit of BTEX migration Figure 3 depicts the distribution of dissolved benzene in the site vicinity MTBE was detected in both MW 15A and MW 15B during both rounds of sampling Figure 4 ' depicts the distribution of MTBE in groundwater in the site vicinity The MTBE clearly originates from offsite sources + Lithological data from the newly installed wells supports the interpretation of two sandy zones of ' significance in the site vicinity (the A and B Sands) The deeper C Sands appear to be very limited in thickness The cross sections depicted on Figures 5 and 6 illustrate the thickening of the A Sand along an axis running approximately along the southern property line of the subject ' site The A Sand appears to pinch out both to the north and south of this axis The B Sand is more laterally widespread but appears to thicken along a similar axis In the vicinity of monitoring wells MW i 1 and MW 12, the B Sand Is at its maximum thickness and, in MW 11, appears to merge with the overlying A Sand These sands have the potential to function as conduits for preferential contaminant transport from west to east and appear to be doing so ' As the data clearly show, a mobile plume of dissolved MTBE originating offsite has migrated beneath the 102 S Wilson Way property Based upon the concentration trends observed in the past, the plume can be expected to continue to migrate at a rapid rate toward the northeast The ' chemical mobility of the oxygenate coupled with its transport down a preferential flowpath • should make it a subject of much greater environmental concern and priority than the relatively static plume of hydrocarbons resulting from the old release at 102 S Wilson Way 1 ❑GZA FSIVOL_IIDATAIGAOUNDZ.ESROEKICLOSITRL\Reports\AM_HSA_Mud wells doc