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16 QUARTERLY MONITORING REPORT USA STATION##65-LODI APRIL,1996 2.1.2 Local Hydrogeology ' Groundwater elevation in S1 and S2 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 Since 1991 the water level has fluctuated between 10 to 13- feet below MSL The direction of groundwater flow and magnitude of the hydraulic gradient across the site have remained relatively consistent flowing southerly Historically the magnitude of the gradient has been between 0 002 and 0 003-feet/feet (ft/ft) A summary of water level records are contained in Table 2 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 bonngs Two of the bonngs (S-A and S-B) were advanced to depths of 20- feet and backfilled The remaining two borings were advanced to depths of approximately 50 5- feet and completed as groundwater monitoring wells (S-1 and S-2) Following well installation, the groundwater levels were recorded at approximately 41 and 40-feet below the ground surface (bgs) in S-1 and S-2, respectively This work was commissioned by Shell Oil Company, which was considering purchasing the site at the time ' In August, 1987, Applied Geosystems completed the installation of three additional groundwater momtonng wells (MW-1, MW-2 and MW-3) The wells were completed to depths of approximately 60-feet bgs Soil samples collected from the bonngs during drilling were non- detect for total petroleum hydrocarbon (TPH) compounds and benzene, toluene, ethylbenzene, and xylenes (BTEX) Groundwater samples collected and analyzed from each boring detected TPH In August, 1988, USA requested that Applied Geosystems resample the wells Analytical results t again detected TVH in each of the wells 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) The wells were completed to ' total depths of approximately 60-feet bgs 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 ' 2