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1.3 STATUS OF PETROLEUM HYDROCARBONS IN THE SOIL AND GROUND- WATER To date, 16 monitoring wells have been installed on the site and 7 off the site to investigate the presence of petroleum hydrocarbons in the soils and groundwater and their source(s) (Figure 4) Because of the drought over the past four years, only eight of the new onsite wells contained water and could be sampled in May 1991 Five offsite wells were also sampled None of the old wells (MW 1-MW7) could be sampled, because water levels were insufficient to obtain represen- tative samples Boring logs for the existing wells are provided in Appendix A ' Liquid-phase hydrocarbons (LPH) have been found in onsite wells RW3 and V4 The highest concentrations of dissolved hydrocarbons were measured in samples of groundwater from wells located in the northern part of the site (RW1, RW2, RW4, V1, and V3) and in the downgradient offsite well MW 10 ' Disregarding the saturation levels that would be expected from the presence of LPH, the highest hydrocarbon concentration found so far was in V4, at 200,000 gg/L of TPH, with 31,000 µg/L of benzene Hydrocarbon concentrations are also high farther upgradient--up to 57,000 µg/L of TPH in well MW8 This area may have a separate source of hydrocarbons and will require more independent investigation The function of the proposed hydraulic barrier is to limit the infiltra- tion of any hydrocarbons from the upgradient area (MW8) Judging from data collected during previous investigations (EA 1987a, 1987b, 1988a, 1988b, ' 1989a, 1989b, 1990, 1991), fuel may have been lost when groundwater was significantly lower than present levels A subsequent increase in the groundwater level may have trapped hydrocar- bons in the soil, this may be why initial investigations identified only localized concentrations of hydrocarbons in the soil above the water table Concentrations of TPH to 6,900 mg/kg were detected in samples from soils excavated from the tank pit, but no elevated concentrations of TPH were detected in soil samples collected in the unsaturated zone in the monitoring wells ' Entrapped hydrocarbons may dissolve in groundwater and disperse in the direction of ground- water flow. Investigations carred out as the water level has dropped have revealed that most of the petroleum hydrocarbons occur between 30 and 40 feet below the surface Part of the petrole- um hydrocarbons were released when the water table dropped below the top of the fuel-saturated soils, as observed previously in wells MW3 and MW6, and now in RW3, where recently as ' much as 0 5 feet of LPH was measured (Table 1) A similar situation is found upgradient of the site, where the highest concentration of petroleum hydrocarbons in soil (3,200 mg/kg TPH) was found below the water table. Such a condition leads to petroleum hydrocarbons actively dissolv- ing in groundwater to the degree of saturation, which hinders both of the natural processes, biodegradation and evaporation, that usually act when petroleum hydrocarbons contact the atmosphere in the vadose zone. Remediation of the site will require- (a) capturing the dissolved hydrocarbon plume and depressing the water table by pumping groundwater, and (b) recovering ' -W-b6 7394Vjd91/M 4