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• 11 April 2003 AGE-NC Project No 01-0865 Page 7 of 9 vadose zone The steep gradient caused by the slough and levee{0 23 ft/ft on Figure 4 of the Condor- prepared Quarterly Ground Water Monitoring Report,dated 03 May 2001)transported the dissolved MTBE eastward to the area east of the ASTs, where Condor's and AGE's ground water maps illustrate gradients ranging from 0 02 to 0 07 ft/ft between MW4 and MW2 This Interpretation is consistent with the following site history- • The USTs were utilized up to circa 1991,well into the age of MTBE usage in gasoline, • The highest petroleum-hydrocarbon impacts to soil are at the former UST location, • 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 at the site for the last few years Contaminants that have migrated to the flat land, where hydraulic gradients are low,would tend to be trapped there,mainly in the peat zone,where porosities are very high,but average permeabilities are very low 5.0. SUNEM ARY AND CONCLUSIONS Based upon data collected dunng this subsurface investigation, AGE concludes the following • The levee material consisted of a dry, brown, poorly-sorted coarse-grained sand underlain by dry to moist,tan to dark brown silty sand with gravel Below approximately 18 feet bsg, the soil beneath the levee and the flat land east of the level consisted of an approximately 15-foot thick, saturated black peat, underlain by an approximately 10 feet of moist to saturated,olive green,well-sorted to silty fine sand Underlying the sandy layer was a 2-foot section of dry to moist, olive green silty clay, followed by a basal silt to the total depth of 26 feet bsg in deepest boring B13 • The high porosity but low hydraulic conductivity of the peat layer suggests that the peat may retard downward-migrating contaminants at the UST location, absorbing much of the contaminants on the upper surface of the layer Differential ground water flow at the top of the peat layer,under a steep hydraulic gradient,may flush out and transport the contaminants down-gradient, however, lateral contaminant migration within the peat layer may be slow due to the low hydraulic conductivity • The moderate to high nutrient concentrations and moderate to high bioenumerations in the peat suggest that petroleum hydrocarbon compounds will potentially decrease over time via natural attenuation Advanced Geoltnvtronmental,Inc