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' Bank of Stockton Page 4 Summer 1996 Quarterly Monitoring Report Project No 102 2 October 10, 1996 ' The samples were analyzed using the following methods ' • method 8020 - benzene, toluene, ethylbenzene, total xylenes (BTEX), • method LUFT/8015m/- total volatile hydrocarbons as gasoline (TPH-Gasoline), ' • method 3510/8015m- total extractable petroleum hydrocarbons (TEPH), The detection limits for the above analyses are listed in Table 2 of Appendix B 2.0 FINDINGS AND DISCUSSION ' The following summary of findings is based on field and laboratory data for this event ' MW-1 In monitoring well MW-1, the BTEX compounds detected from the sample of September 3, 1996 were benzene (5 ppb), toluene (12 ppb), ethylbenzene (9 ppb) and xylene (41 ' ppb) The total detectable TPH was found to be 245 ppb These levels compare to 0 TPH to the March sampling event and 0 9 ppb of benzene It was also noted that the PID reading was 85 ppm at the well head In the re-sample collected on September 16, 1996 (2 weeks later) the BTEX levels were benzene (2 ppb), toluene (0 6 ppb), ethylbenzene (ND) and xylene (0 9 ppb) The total ' detectable TPH was found to be 74 ppb The groundwater level to MW-1 was observed to have fallen about 4 inches in the two week period It was also noted that the PID reading had fallen to 5 ppm at the well head The recent low contaminant levels detected in the March 28, 1996 event corresponded j with the highest recorded stand of groundwater since monitoring began in 1992 That ' level, some 4 feet higher than to this present event, is also several feet above the highest slots in the well casing It is 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 3id levels After reviewing all available information, it is now suspected that after two years of heavy rainfall, the groundwater rose to levels higher than those of recent years and to 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 The results of the re-sample event suggest groundwater is continuing to fall rapidly and the newly mobilized hydrocarbons are rapidly dispersing and degrading If this is the case, the next monitoring event should 2937 Veneman Ave , #B240 Phase One Environmental Inc Phone (209) 569-0293 Modesto, CA 95356 Fax (209) 569-0295