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Cecchini, Cecchint, and Giovannoni, Fueling Island Horizontal continuity of the sand is indicated by sand layers at 19 feet and at 64 feet in the exploratory borings for UST-2, UST-3, and UST-4 (FSE, 1995b) Concentrations of TPH-D in soil were variable with depth with the highest concentration of 16,000 mg/Kg at a depth of 20 feet bgs (where the sand was first encountered) Concentrations of TPH-G and TPH-D decreased to low to non-detect levels at a depth of 35 feet bgs, but are at elevated concentrations from 40 feet to 45 feet bgs TPH-G and TPH-D concentrations decrease to non-detect levels at 62 feet The soil plume extends to a depth of approximately 50 feet bgs (-20 feet MSL) and is of limited extent horizontally as indicated by non-detect results in the soil samples from UST-2, UST-3, and UST-4 (ESE, 1995b). Generally, ground water does not rise above -20 feet MSL in this area indicating that only minor leaching of low levels of hydrocarbons would occur under present conditions 4.4.2 Ground Water The average ground water elevation calculated for the site has decreased approximately three feet relative to that reported for the First Quarter, March 1996 (ESE, 1996) T his seasonal decrease is likely due to the drier warmer weather and regional pumping from agricultural wells The second quarter findings indicate a flow direction to the west at a magnitude of 0.004 foot/foot. Ground water findings in monitoring well UST-5 indicate that no free-floating product was present and that the concentration of TPH-D and TPH-G were relatively low at 110 ttg/l and 180 µg/1, respectively Benzene was found to be slightly elevated at 8 8 µg/1 in UST-5 The only other well with ground water findings was UST-2 with TPH-D reported at 140 µg/l Relatively low TPH-D concentrations in UST-2 were previously reported in the Fourth Quarter, 1995 (ESE, 1995d) 4.4.3 Conceptual Remediation Plan This section briefly describes the conceptual remedial action to cleanup hydrocarbon- impacted soils in the fuel island area The proposed remedial action would use the excavation to clean up the shallow unpacted soil zone (surface to 25 feet hgs) and air infection to stimulate bioremedial cleanup of the deeper hydrocarbon-impacted soil zone (25 feet to 50 feet bgs) The shallow phase of cleanup would follow the retrofit of the F 16M4000QM96 RPT 7 Environmental Science&Engineering, Inc