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I FIRST QUARTER, 1998 ' GROUNDWATER MONITORING REPORT ! USA STATION#65-LODI ' FEBRUARY,1998 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 in 1989 By September, 1991, the water level elevations had dropped to approximately 13- feet below MSL Groundwater elevations have since recovered to 1989 levels or higher The direction of groundwater flow and magnitude of the hydraulic gradient across the site have t remained relatively consistent;flowing-southerly at a=gradient-betweenm0 002'antl°0 004466t/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 dnUmg 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 installed three additional groundwater monitoring wells (MW-1, MW-2 and MW-3) USA requested that Applied Geosystems complete four additional , monitoring wells (MW-4, 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 1t 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 ' On March 14, 1995, WEGE completed a one day vapor extraction treatability test, 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 The Park Corporation(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 In July, 1997, GBH was retained to provide environmental monitoring and remediation at the .} subject site 2