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August 4, 1993 Working To Restore Nature Fire Engine Company No 2, Stockton, Califorrua GROUNDWATER MONITORING On May 25, 1993, RESNA measured the water levels in wells MW-1 through MW-4 and RW-1 and collected groundwater samples from the wells for subjective and laboratory analyses The field methods used are presented as an attachment to this report Floating product or product sheen was not noted on samples from the wells (Table 1) The depth to water measurements and the elevations of the well casings (Table 1) were used to evaluate the groundwater flow direction and gradient beneath the site (refer to attached Field Procedures) The groundwater gradient beneath the site ranged from / approximately 0 01 and 0 006 with flow directions to the northwest and northeast (Plate 3) t, Since the previous monitoring event, the groundwater surface has risen approximately 2 02 feet to an average elevation of 40 7 feet bsg The direction of groundwater flow has ranged from southeast to northeast since groundwater monitoring was initiated in December 1988 Groundwater samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg) and as diesel (TPHd) using EPA Method 8015 (modified for gasoline and diesel) and for benzene, toluene, ethylbenzene, and total xylene isomers using EPA Method 602 These samples were analyzed at Sequoia Analytical in Sacramento, California (California Department of Health Services Hazardous Waste Testing Laboratory Certification No 1624) Copies of the Chain of Custody Records and laboratory analysis reports are attached Results of laboratory analyses indicated dissolved TPHg was detected only in samples from wells MW-1 and RW-1 at concentrations of 2,200 and 450 parts per billion (ppb), respectively Benzene was also detected in wells MW-1 and RW-1 at 570 and 4 7 ppb, respectively Concentrations of TPHd ranging from 60 (MW-3) to 310 ppb (MW-1) were reported for wells MW-1, MW-3, and RW-1 The laboratory report indicates that the TPHd reported for MW-1 and RW-1 contained hydrocarbons less than C1z Results of laboratory analyses are summarized in Table 2 SUMMARY AND RECOMMENDATIONS The groundwater flow direction and gradient were generally consistent with those evaluated during previous events Dissolved gasoline hydrocarbons were detected in samples from wells MW-1 and RW-1 with concentrations generally consistent with previously measured values Dissolved diesel hydrocarbons were reported for wells MW-1, MW-3, and RW-1 during this sampling event, however the reported carbon range (less than C1z) suggests that these hydrocarbons may actually be gasoline or a mixture of gasoline and diesel fuel o622jesc J 3soo¢4 2