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' Bank of Stockton Page 4 Autumn 1996 Quarterly Monitoring Report Project No 102 2 December 27, 1996 ' petroleum hydrocarbons were detected It was noted at that time that the recent low contaminant levels detected in the March 28, 1996 event corresponded with the highest ' recorded stand of groundwater since monitoring began in 1992 That level, some 4 feet higher than in the September event, was also several feet above the highest slots in the well casing It was also noted that at the time of the re-sample event of September 16, ' only two weeks later, groundwater had fallen an additional 4 inches, and contaminant, and PID levels were much reduced from the September 3 levels ' After the September event, it was postulated that two years of heavy rainfall had caused the groundwater to rise to levels higher than those of recent years, and in so doing, came into contact with soil containing residual petroleum hydrocarbons that had not been ' previously wetted As the groundwater levels fell to an elevation below the top casing slots, the newly mobilized hydrocarbons were then able to enter the well and become detectable at elevated concentrations It was suspected at that time that the levels would ' rapidly fall off to levels observed in earlier sampling events The results of this current round of monitoring appear to support that hypothesis ' MW-4 The only petroleum constituent detected in this well was benzene which was found to be present at a level of 0 4 ppb The most recent previous sample was collected on March 26, 1996 when benzene was found to be present at a level of 0 5 ppb ' MW-5 No detectable petroleum hydrocarbons or BTEX compound were found in the sample ' from this well The most recent prior sample was collected on November 29, 1995 when 110 ppb of oil range hydrocarbons were detected It was postulated at that time that the I oil was a result of cross-contamination from automotive crankcase oil The well is situated in the Cutter Lumber parking area These sample results appear to be consistent with that hypothesis 3.0 CONCLUSIONS The following conclusions have been drawn regarding this quarterly groundwater sampling event i • The direction of groundwater flow and gradient are consistent with prior monitoring events The level of groundwater has risen about one foot, and the well slots are again submerged several inches i2937 Veneman Ave ,#B240 Phase One Environmental Inc Phone (209) 569-0293 Modesto, CA 95356 Fax (209) 569-0295 i