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THIRD QUARTER,1997 GROUNDWATER MONITORING REPORT USA STATION#65-LODI AUGUST,1997 2.3 Local Hydrogeology Groundwater elevations in S-1 and S-2 were approximately 1-foot below MSL when the wells were drilled in 1987. The groundwater elevations at the site ranged between 5 and 6-feet below MSL from January 20, 1989 to April 18, 1989. By September, 1991, the water level elevations had dropped to approximately 13-feet below MSL. Groundwater elevations have since recovered to 1987 levels or higher. The direction of groundwater flow and magnitude of the hydraulic gradient across the site have remained relatively consistent, flowing southerly at a gradient -- between 0.002 and 0.004-feet/feet (ft/ft). Water level records are summarized in Table 1. 2.4 Summary of Previous Investigations -� A limited site investigation completed in March, 1987 detected petroleum hydrocarbon compounds in the soil and groundwater beneath the site. The investigation included the drilling and sampling of four soil borings, two of which were completed as groundwater monitoring wells (S-1 and S-2). In August, 1987, Applied Geosystems completed the installation of three additional groundwater monitoring wells (MW-1, MW-2 and MW-3). USA requested that Applied Geosystems complete four additional monitoring wells(MW4, MW-5, MW-6, and MW-7) in the downgradient direction to further define the lateral extent of the impacted groundwater. Applied Geosystems then prepared a Problem Assessment Report (PAR) after completing monitoring wells MW-6 and MW-7. The PAR concluded that petroleum hydrocarbon impacted water had migrated beyond the property boundaries, and that the groundwater in the upper aquifer, above 42-feet below ground surface (bgs)was not in hydraulic continuity with the deeper aquifers. It was their opinion that further delineation of the impacted soil and water was not required. On February 8, 1993, Western Geo-Engineers (WEGE) completed monitoring well MW-8 to a depth of approximately 60-feet bgs. The well is downgradient approximately 8-feet southeast of the tank field. This well has consistently contained total petroleum hydrocarbon(TPH), benzene, toluene, ethylbenzene, and xylenes (BTEX) compounds. On March 14, 1995, WEGE completed a one day vapor extraction treatability test. The test was performed using monitoring wells MW-4, S-1, S-2, and MW-8. Test results are summarized in the WEGE report"Results of Pilot Test for Vapor Extraction', dated April 17, 1995. _ USA retained Park to prepare a Remedial Action Plan(RAP). The rationale and design of the necessary environmental remediation and sampling activities at the site were presented in Park's November, 1995 RAP and in a February 28, 1996 letter to the County. 2