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PRIM, Stockton Scavenger Assoctation Facility November, 1997 Page 2 2 0 BACKGROUND The following discussion of the site background is based upon information contained in the Preliminary Closure Report, June 1994, prepared by The MARK Group, Inc , (MARK) In May 1989, EMCON Associates (EMCON) installed three groundwater monitoring wells at the subject site to assess the presence of petroleum hydrocarbons in groundwater EMCON reported that groundwater was approximately 23 feet below ground surface (bgs) and flowed in a northwesterly direction No petroleum hydrocarbons were detected in groundwater during the Imonitoring event In September 1992, Parker Environmental Services excavated and removed two diesel fuel ' underground storage tanks (USTs) with capacities of 10,000 and 7,500 gallons, and one gasoline UST with a capacity of 7,500 gallons Analysis of sod stockpile samples and sod samples collected from the UST excavation indicated the presence of petroleum hydrocarbons On November 5, 1992, MARK performed a preliminary assessment of soil and groundwater in the vicinity of the former USTs The assessment included the advancement of five bonngs (B-1 through B-5) to depths of 25 to 45 feet bgs The analytical results for the samples indicated that soil and groundwater beneath the SSA site had been impacted by petroleum hydrocarbons Results of this assessment are documented in the MARK Sampling and Analysis Report(December 1992) On February 3, 1993, MARK decommissioned the three groundwater monitoring wells installed by EMCON because of the location of the well screen intervals and the condition of the wellhead boxes The EMCON wells penetrated a groundwater-confining clay layer that exists at a depth of ' 41 to 45 feet below ground surface (bgs) On May 24 and 25, 1993, MARK installed three new monitoring wells (MW-1, MW-2, and MW-3) at the locations shown on Figure 2 On October 14, 1993, at the request of the San Joaquin County Public Health Services, Environmental Health ' Division (EHD), MARK installed monitonng well MW-4 at the location shown on Figure 2 The groundwater monitoring wells MW-1 through MW-3 have been monitored quarterly since installation in May 1993 Well MW-4 has been monitored quarterly since its installation in October ' 1993 Since installation of these wells, depths to groundwater have varied from approximately 18 to 32 feet bgs In general, the groundwater elevations at the SSA site have risen more than 10 feet since November 1994 Since May 1993, the groundwater flow direction has varied from northwest ' to east-northeast, but has generally been east-northeast The ground water gradient calculated from July 1997 data is 0 01 feet per foot ' Petroleum hydrocarbons have not been detected in MW-1 In MW-3, TPHg has been detected up to 0 11 milligrams per liter (mg/1), TPHd has been detected up to 0 19 mg/l, and benzene has been detected up to 0 0016 mg/l In MW-4, TPHd has been detected up to 0 34 mg/l, and TPHg and benzene have not been detected Except for TPHd, petroleum hydrocarbons have not been detected in wells MW-3 and MW-4 for the last four quarters In MW-2, TPHg has been detected at up to 9 9 mg/l, TPHd has been detected up to 1 1 mg/l, and benzene has been detected up to 2 8 ' mg/I The highest concentrations were detected following monitoring well installation in May 1993 ART 5027002