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t'} U i . 06 March 2002 AGE-NC Project No 01-0865 Page S of 9 The low MTBE concentrations in the former UST area may be an effect of the elevated location of the former USTs Meteoric waters and seepage from the Little Connection Slough, with a tidal influence,may have flushed MTBE from the shallow vadose zone The steep gradient caused by the slough and levee(0 23 ft/ft on Figure 4,Condor's 03 May 2001 Quarterly Ground WaterMonitorcng Report,included as Appendix C of this report)transported the dissolved MTBE eastward to the area east of the ASTs, where Condor's map illustrates an approximate gradient of 0 025 between MW4 and MW2 (AGE found a 0 07 gradient between MW4 and MW2 in December 2001) The reduced gradient in the flat area may retard additional rapid MTBE migration, but the plume does extend a considerable distance eastward The known site history is consistent with the hypothesis of a UST-origin for the dissolved plume • The USTs were utilized up to circa 1991, well into the age of MTBE usage in gasoline, • There have been no documented releases of fuels from the current AST system, • There have been no repairs of the AST system since inception of use, and • MTBE-free fuels have been utilized on the site for the last few years i Taken as a whole, the available data suggests to AGE that the dissolved contaminant distribution encountered at the Herman & Helen's Manna site can be very reasonably interpreted to represent a partitioned plume originating from the former UST area and that the current AST system may not have contributed significantly to the plume 6.0. SUMMARY AND CONCLUSIONS Based upon data collected during this subsurface investigation, AGE concludes the following • Except for borings on the levee,generally,black,moist,peat was encountered in the eastern borings from surface grade to approximately 10 feet bsg Grayish green, moist to saturated, sand to silty sand intervals were encountered from 10 feet bsg to approximately 20 feet bsg, from 24 feet to 26 feet bsg and from 34 to 39 feet bsg Dusky green,damp to very moist, silt and clay were encountered at all other depths • The soil samples collected from boring B7 at 15 feet and 25 feet were impacted by petroleum hydrocarbons However, concentrations of TPH-g and toluene detected in the 25-foot soil sample werejust above laboratory detection limits Based on this and previous investigations at the site the lateral and vertical extent of impacted soil has generally been defined • Ground water samples collected from first encountered ground water were impacted by TPH-g m bonngs B 1,B3, B4,B7, B9 and B 10 Samples collected from deep groundwater,