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1 FORTH QUARTER 1996 QUARTERLY MONITORING REPORT USA STATION #65 - LODI NOVEMBER, 1996 2.1.2 Local Hydrogeology ' Groundwater elevations in S-1 and S-2 were approximately 1-foot below MSL when they were drilled in 1987 The groundwater elevations at the site ranged between 5 and 6-feet below MSL from January 20, to April 18, 1989 By September, 1991 the water level elevation had dropped to approximately 13-feet below MSL Groundwater elevations have since recovered to 1987 ' levels The direction of groundwater flow and magnitude of the hydraulic gradient across the site have remainedrelatively—consistent southerly ata gradient between 0 002 and-4 003— feet/feet (ft/ft) A summary of water level records are presented in Table 1 2.2 Summary of Previous Investigations Results of a limited site investigation completed in March, 1987 detected petroleum hydrocarbon compounds in the soil and groundwater beneath the site The investigation included drilling and sampling of four soil borings two of which were completed as groundwater 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) To further define the lateral extent of the impacted groundwater, USA requested that Applied Geosystems complete four additional monitoring wells in the downgradient direction(MW4, MW-5, MW-6, and MW-7) Applied Geosystems prepared a Problem Assessment Report (PAR) after completion of monitoring wells MW-6 and MW-7 The PAR concluded that petroleum hydrocarbon impacted water had migrated beyond the property boundaries, and it was their opinion that the groundwater in the upper aquifer, above 42-feet was not penetrating the deeper aquifers It was also their opinion that further delineation of the impacted soil and water was not required at that time On February 8, 1993, monitoring well MW-8 was completed to a depth of approximately 60-feet below ground surface (bgs)by Western Geo-Engineers (WEGE) The well is downgradient from the suspected source and 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 on 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 2