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the gasoline USTs. TPHg was not detected in the groundwater from boring SB7, which was located immediately northwest of the former USTs. In addition, floating product was detected L in borings SB2 and FPB-2. Similarly, as indicated on Figure 7, the plume of benzene is north and northwest of the former location of the gasoline USTs. Benzene was not detected in the groundwater from boring SB7, which was located immediately northwest of the former USTs. The extent of contamination to the south is defined by well MW1 and borings SBI and SB7, and the extent of contamination to the north is defined by borings SB3 and SB5. The extent of contamination to the west and the east are inferred and may require additional investigation. Groundwater analytical results from the subsurface investigation and the third quarter 1996 monitoring event are presented on Table 2. �I Borings SBI and SB3 are considered to be outside the petroleum hydrocarbon plume. However, toluene was detected at, concentrations just above the detection limit in the groundwater from both borings but at concentrations well below the maximum contaminant I f level of 100 ppb. In addition, MTBE was detected at low concentrations in SB3. These low concentrations mark the extent of the petroleum hydrocarbon plume, and thus, these borings are considered outside the plume. Dissolved petroleum hydrocarbons beneath the site appear to attenuate rapidly with distance from the central portion of the site as evidenced by analytical data summarized in Table 2. This is most likely due to sorption processes directly related to the Percent organic matter , levels in the aquifer material and the clayey soil of the capillary fringe. As previously stated, percent organic material (total organic carbon) of samples collected from the aquifer material p q at various depths from selected soil borings range from 19% to 23%. Petroleum hydrocarbon contaminants partition between the aqueous phase and the soil matrix and are controlled by the organic carbon content in soils (McAllister and Chiang, 1994). In addition, petroleum hydrocarbons readily adsorb to clayey soil. C Based on the native state effective water conductivity results for saturated zone soil, a prediction of plume movement can be obtained. The 11 to 12 foot (sandy clay)sand sample has a P Y) native state effective water conductivity of 4.55 x 10.7 cm/s, which calculates to 0.47 feet/year. The 15.5 to 16 foot sample (poorly graded sand) has a native state effective water conductivity of 1.51 x 10 cm/s, which calculates to 1,562 feet/year. Taking into account the gradient at the site (0.003 ft/ft) petroleum hydrocarbons floating on the water table in the sandy clay portion of the aquifer will travel by advection alone 0.0014 feet/year, and any I w:195122TIumerpt l l { SMTN