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. 06 March 2002 AGE-NC Project No 01-0865 Page 7of9 AGE believes the available data may actually represent a partitioned plume of impacted ground water It is well known that the various fuel components have differing migration rates in ground water,that MTBE has the most rapid migration rate due to its low adsorption potential for inorganic minerals, and that the BTEX compounds migration rates are less than MTBE Benzene commonly has the most rapid migration rate for a hydrocarbon, and often is the first compound detected at the leading edge of a dissolved hydrocarbon plume, xylenes typically are the slowest moving aromatic compound and lag behind near the release point The monitoring data from MW-4,near the former UST site and the only well consistently yielding benzene and xylenes,may actually demonstrate the principal The highest benzene concentrations in the well were encountered during the period from Mach 1998 to June 1999,the highest xylenes concentrations have been detected since approximately September 2000 to present (December 2001) Toluene concentrations have always been low or undetectable, ethylbenzene may have had peak concentrations from December 1997 to September 1999, slightly ahead of the inferred benzene peak concentration period The aliphatic hydrocarbons making up the bulk of TPH-g have varying migration rates that depend on chain-size and configuration,but are generally slower than benzene The eastern part of the plume is in a flat area adjacent to agricultural land, application of fertilizers to the agricultural land may have stimulated bacterial growth which has in turn selectively degraded any volatile aromatic compounds, especially benzene, that reached the area The former UST pit was located on the levee, in an elevated position relative to the ASTs and fields located toward the east, and slightly elevated relative to the water body on the west Ground water flow has been toward the east, at a higher gradient in the area near the levee and lower gradient toward the east Mapping the dissolved fuel component concentrations shows high MTBE and TPH-g concentrations east of the AST site and elevated BTEX concentrations in the former UST area Although the data set is sparse (three ground water sample points), dissolved benzene and total xylenes were present in samples collected from the UST area(B7) extending to the southeast of the AST area(DP-5) In this area where the aromatics are present, the highest benzene concentration was detected in DP-5 (the leading aromatic hydrocarbon plume9), the highest total xylenes concentration was in MW-4 (trailing plume?) Figures 7 and 8 illustrate the approximate extent of dissolved benzene and xylenes, respectively, in shallow ground water The core of high MTBE concentrations hes toward the ease and northeast of the dissolved aromatic compounds plume (leading edge?) Although the data set is not robust, AGE believes the contaminant distribution could be interpreted as a partitioned plume of fuel constituents, with the MTBE plume nearly completely detached from the source area The unusual aspect of the dissolved contaminant distribution is the high TPH-g concentrations also located northeast and east of the dissolved aromatic hydrocarbon plume,similar to the MTBE distribution may be related to selective bacterial degradation of aromatic hydrocarbons, as noted above