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09 August 2000 AGE-NC Project No. 95-0106 Page 8of10. • As can be seen on,Table 2, TPH-g concentrations were generally'higher in ground water ' samples collected from the offsite well MW-4, than in the.on-site well, MW-1,'however, benzene concentrations have generally.been higher in samples from MW-1. Continued detection of TPH-g and BTEX in ground water samples collected from the off•-site well MW4 indicates that the plume of impacted.ground water has migrated off--site towards the north, however, the general lack of detection of hydrocarbons in the MW-6 and MW-5 wplume ells shows that the is not wide-spread and has not migrated northeast of MW-4 or northwest of the former•UST pit. • The extent of the plume around MW-4 is.constrained by data from MW-6 toward the north- northeast, MW-5 toward the southwest and P4 toward the southeast relative to MW-4. The northwestern extent of the plume from MW-4 is not well constrained. The results of this grid previous monitoring events and previous soil investigations leads AGE to ' conclude that: • The hydrocarbons encountered in ground water samples from MWA and MW-4 represent' , - a northward extension-of the hydrocarbon-impacted groundwater that had already reached ' its current extent at the time the monitoring well system.was installed beginning in 1996. - !� g Additional migration of the plume since installation of the.monitoring system has not been ' detected: • The apparent gradual decline of hydrocarbon concentrations in samples collected from. MW-4 may, be the result of natural attenuation processes on the 'stabilized? plume; the' ' variationof hydrocarbon concentrations in samples collected'from MW-1 may be due to its - closer proximity to impacted,soil.left in place near theTormer.UST site. ; • 'hilpacted ground water is probably limited to the more porous and permeable sandy,interval' ' underl in the site at a depth of approximately 10 feet to 25 feet bs '; the silt /cla a unit' YI g . P PP Y 9 Y. Y Y • ' overlying the sandy zone has lower permeability and higher surface area per'unit volume. ' The fine-grained,`shallow,'soil is likely to adsorb and retard the Iateral movement of the e petroleum hydrocarbons. • Assuming a plume of impacted ground water extending 180 feet north-south and 110 feet east-west, as shown on Figure 5, the plume is confined to the more sandy interval from 10 -feet to' 25.. feet bsg, the sand has 25 'percent effective porosity, and, that the average concentration of TPH-g is 1187 gg/l and the average benzene concentration is 164.1 '. I-tg/I(Both the average for MW-1 and MW-4 4or last five events approximately )� then PP Y .. 1651030.9 liters -of water are`impacted, yielding approximately 2 kilograms of gasoline (0.704 gallons)and 0:27.1 kilograms of benzene are currently impacting the ground water. 1 Appendix C shows the calculations. ' Advanced GeoEnvironmental;Inc.