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April 27, 199311X5MA • ARCO Station No 548, Stockton, California Working To Restore Nature drilling of eleven soil borings and construction of four vapor-extraction wells and four groundwater monitoring wells in those borings In October 1992, technician from RESNA sampled these four monitoring wells for laboratory analyses Results indicated that dissolved hydrocarbons were not detected in ground water The ground-water flow direction was northwest with an approximate hydraulic gradient of 0 02 (RESNA, Report in preparation) GROUND-WATER MONITORING On February 24, 1993, a technician from Emcon took water-level measurements and collected ground-water samples from monitoring wells MW-1 through MW-4 for subjective and laboratory analyses On March 1, 1993, a technician from RESNA measured depths to water and collected ground-water samples for subjective analyses The technician did not observe floating product or product sheen on samples collected from the wells (Table 1) The surveyed elevations of the top of casing and the depth-to-ground-water measurements (summarized in Table 1) were used to evaluate the potentiometric surface beneath the site On February 24, 1993, the direction of ground-water flow was northwest with a gradient of approximately 0 01 (Plate 3) The direction of groundwater flow has generally been northwest since our investigation began The groundwater surface has risen an average of • 14 feet since the last quarterly monitoring event Ground-water samples from each well were analyzed for total petroleum hydrocarbons as gasoline (TPHg) and benzene, toluene, ethylbenzene, and total xylene isomers (BTEX) using Environmental Protection Agency Method 5030/8020/DHS/LUFT Method The samples were analyzed at Columbia Analytical Services, Inc in San Jose, California (Hazardous Waste Testing Laboratory Certificate No 1426) The results of analyses are summarized in Table 1 Copies of the laboratory analysis reports are attached Results of analyses indicated TPHg and BTEX concentrations were below laboratory detection limits in ground-water samples collected from wells MW-1 through MW-4 i WORK IN PROGRESS RESNA is currently preparing a Preliminary Investigation and Evaluation Report that will be submitted during the second quarter 1993 i CONCLUSIONS AND RECOMMENDATIONS The ground-water flow direction has consistently been northwest since this investigation began Gasoline hydrocarbons were not detected in ground-water samples collected from monitoring wells MW-1 through MW-4 0375ccar 30008-7 2 1