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1 FOURTH QUARTER,1997 , GROUNDWATER MONITORING REPORT • USA STATION#65-LODI ' NOVEMBER, 1997 2.3 Local Hydrogeology Groundwater elevations in S-I and S-2 were approximately I-foot below MSL when the wells ' were drilled in 1987 The groundwater elevations at the site ranged between 5 and 6-feet below MSL in 1989 By September, 1991, the water level elevations had dropped to approximately 13- 1 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 t 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 P g 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 r aquifer, above 42-feet below ground surface (bgs)was not m 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 approximately 8-feet downgradient and southeast of the tank field This well has consistently contained total petroleum hydrocarbon(TPH), benzene, toluene, ethylbenzene, and xylenes (BTEX) compounds 1 On March 14, 1995, WEGE completed a one day vapor extraction treatability test The test was performed using monitoring wells MW4, 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 ,