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Page 5, c111,,-10l17/94 available on the north-south extent of contamination Although the lateral limits cannot be defined, the vertical extent has been reasonably established The data convincingly demonstrate that the hardpan layer between 60 and 65 feet restricted downward percolation of hydrocarbons and limited the contaminated zone to the 30-60 foot depth ' interval Only diesel has been detected in soil samples below this depth and concentrations were low 5.2 Extent of Groundwater Contamination The concentration of diesel-range compounds is also shown schematically in Figure 5 Well ' MW-1 appears to be located between two contaminated "pools", which may be connected out of the plane of the diagram However, the location of the three wells precludes mapping the ' groundwater plume in plan view, and its lateral extent is unknown It is interesting that GT-2 has contamination in the groundwater, whereas MW-1 does not, even ' though MW-1 is closer to the source of contamination Further, hydrocarbon concentrations in the water sample from GT-1 are much lower than those from GT-2 However, because the ' groundwater elevation is essentially the same in all wells, all three wells are slightly upg►radient of the tank cavity at this time 6.0 CONCLUSIONS AND RECOMMENDATIONS The available data indicate that groundwater beneath t the site has been impacted by an unauthorized release of hydrocarbons, and that residual soil contamination is sufficient to pose a r continued threat to groundwater if the groundwater level rises above the hardpan layer in the future Field observation, in conjunction with laboratory data, also indicates that the soil beneath ' the tank excavation contains high concentrations of hydrocarbons Due to the limited number of sample borings, the extent and volume of contaminated soil cannot be determined at this time However, it is evident that this volume exceeds 750 cubic yards Excavation to remove the ' contaminated soil would reqwre the removal and stockpiling of a similar or larger volume of uncontaminated overburden Furthermore, excavation to a depth of 55 feet is not feasible ' Therefore, Upgradient concludes that remediation of soil contamination at this site cannot be achieved by conventional excavation techniques Although hydrocarbons are present in groundwater at the site, concentrations are relatively low Contaminant concentrations are also low in the vadose interval below the "hardpan zone" As pointed out in section 5 1, it appears that flaws in the hardpan (fractures, thin or nonexistent area, etc ) may have allowed contamination to reach groundwater Groundwater may have been contaminated prior to 1987, when the water table was above the hardpan The subsequent drop 1