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(Figure 2) were selected for laboratory analysis, and all five contained both diesel and gasoline- range hydrocarbons. Gasoline concentrations decreased downward from 550 mg/kg at 35 feet to 6.5 mg/kg at 55 feet, and diesel concentrations decreased downward from 6,500 to 98 mg/kg. BTEX compounds also decreased downward and were not present in the bottom sample. Then; was less evidence of contamination in soil samples from GT-2, and only three samples were selected for analysis. No contamination was detected at 50 feet or 60 feet, but gasoline was dectected at 38 mg/kg and diesel was detected at 250 mg/kg at 55 feet. Subsequent drilling demonstrated that these hydrocarbons were trapped in low-permeability silt and cemented sandstone (hardpan)just below the base of a permeable and coarse-grained channel sand bed. Further drilling took place in late 1996. Upgradient installed three new monitor wells and three additional soil borings, and prepared a Problem Assessment Report in December 1996. Based on the data collected from these borings, Upgradient concluded that the diesel and gasoline had leaked vertically downward through the upper silt bed into the underlying channel deposit, and then spread laterally as well as downward until reaching the base of the channel, where the hydrocarbons became ponded because they could not easily penetrate the silt bed and hardpan layer that underlie the channel. Hence, during the drought years of the early 1990's, the groundwater table was below 75 feet and had little or no contact with contaminated soil at the base of the channel sand. However, once the water table rose above 60 feet, previously adsorbed hydrocarbons became dissolved into the groundwater and groundwater concentrations increased. As the water table continued to rise above the screened intereval in the wells, concentrations decreased again. Upgradient sugested a few options for dealing with the soil and/or groundwater contamination, but due to lack of interest by the client and the regulatory agencies,no work was requested or performed. 3.0 PROPOSED ASSESSMENT Two borings will be drilled with a hollow-stem auger drilling rig. The first boring will be drilled to an estimated depth of 60 feet and completed with 25 feet of well screen so that groundwater samples can be collected when the water depth is shallower than it was when the previous: six wells were installed. The proposed location for the well (GP-10) is shown in Figure 2. Because this location would be suitable for groundwater extraction or other treatment that mi het be performed_at.? a future date, the well will be completed with 4-inch-diameter casing Details of the well construction are shown in Figure 3. To provide additionat Ita tor mapping the extent of soil contamination, this boring will be sampled at 5-foot intervals from 10 to 35 feet, and will be continuously sampled from 35 to 60 feet. The samples will be screened for the presence of volatile hydrocarbons using a Photovac Microtip HL-2000 photo-ionization detector, described on the boring log, and then preserved for laboratory analysis. The cored interval will be photographed and archived. Drill cuttings will be placed on plastic sheeting for future disposal. Boring 4GP-11 will be drilled within the former tank excavation, between TB-1 (where diesel was first detected at a depth of 70 feet) and GT-3 (where the low [3.4 ppm] concentrations were detected between 60 and 85 feet). As required by PHS/EHD, this boring will be drilled to 120 feet, which is believed to be the lowest water depth since the USTs were installed in 1974. This area consists of imported fill material between the surface and 35 feet, and sand from 35 to 50 feet.