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1 SECOND QUARTER, 1997 ' GROUNDWATER MONITORING REPORT i USA STATION#65 -LODI t JUNE, 1997 2.3 Local Hydrogeology ' Groundwater elevations in S-1 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 from January 20, 1989 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 sitehave remainedrelatively consistent;flowing southel'ly at a-gradnenfbetween40-002mand 0 004-feet/feet (ft/ft) A summary of water level records are presented in Table 1 , 24 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 gioundwater monitoring wells (MW-1, MW-2 and MW-3) USA requested that Applied Geosystems complete four additional monitoring wells (MW-4, 1\4W-5, MW-6, and MW-7)_m the downgradient _r 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 Plot 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