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I S. SUMMARY AND CONCLUSIONS Twelve monitoring and extraction wells were installed on and off the site of former Exxon RS 7-3942. Four of them(Vi-V4)will, together with some of the old wells which are presently dry like 1v1W5 and MW6, be used for soil vapor extraction. Wells VI and RW4 are planned to be used for both groundwater and soil vapor extraction. New downgradient wells MW11 and MW12 were installed about 600 and 230 feet from the site,respectively (Figure 2),and will be used to monitor groundwater quality in this direction. One injection well (lW 1) was installed upgradient; this well,together with older monitoring wells MW1, MW4,and MW7,will be used to reinject treated water back into the aquifer,thereby creating a hydraulic barrier for an upgradi- ent plume to prevent it from moving under the site. Two piezometers(PI and P2)were installed to monitor the effectiveness of water reinjection in raising water levels in this area. To monitor effectiveness of soil vapor extraction, two cluster vadose zone piezometers (PSV 1 and PSV2) were installed. They were screened at two levels: in the upper sandy zone between 12 and 17 feet below the surface and at the lower sandy zone between 30 and 35 feet. Analysis of air samples collecte3 periodically from these wells will monitor the effectiveness of the soil vapor extraction system. Analysis of soil samples collected at depths from 20 to 40 feet from newly drilled boreholes confirmed that lost product remains below the present water table,under and downgradient of titre site. At the former tank field, soil containing petroleum hydrocarbons extends about 10 feet above the water table. LPPH has been recently measured in wells V4 and RW3 and has been measured in the past in wells MW3 and MW6,defining the lateral extent of LPPH. The maximum thickness of LPPH has not exceeded 1 foot. Analytical results of the groundwater samples indicate that a dissolved petroleum hydrocarbon plume extends 100-150 feet downgradient of the site. Samples collected from well MW 12 (230 feet downgradient)and well MW 11 (about 600 feet downgradient)do not contain concen- trations of petroleum hydrocarbons greater than detection limits. Sharp increases in petroleum hydrocarbon concentrations in well MW8,immediately downgradi- ent from the Shell service station, suggest that an upgradient source may be contributing tv the dissolved petroleum hydrocarbon plume. The water level is dropping at a rate of about 3 feet a year, but the flow direction to the east- northeast remains unchanged. �6:l..gnvaLi,m9�h. 13