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®M ®VA FEAS November 8, 1993 Working to Restore Nature ARCO Station No. 548, Stockton, California • RESNA began monitoring the groundwater in wells MW-1 through MW-4 on October 30, 1991 (Applied GeoSystems, November 3, 1989). After installation of well MW-5, the well was added to the monitoring program. Dissolved hydrocarbons have been detected in monitoring wells MW-1 and MW-5 since monitoring began. • GROUNDWATER MONITORING EMCON of Sacramento, California, was contracted by ARCO to conduct the field work at the site; EMCON subsequently forwarded their field notes and the laboratory reports to RESNA. Field work included measuring depths to water and collecting groundwater samples for subjective and laboratory analyses. RESNA used these data to evaluate the direction and gradient of groundwater flow beneath the site. The locations of the monitoring wells are shown on Plate 2. On September 2, 1993, a technician from EMCON measured the depth to water and collected groundwater samples from monitoring wells MW-1 through MW-5 for subjective and laboratory analyses. The field methods and procedures used are presented in an attachment to this report. 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-groundwater measurements (summarized in Table 1)were used to evaluate the potentiometric surface beneath the site. On September 2, 1992, the direction of groundwater flow was southeast with a gradient of approximately 0.01 (Plate 3). The direction of groundwater flow has generally been southeast since our investigation began. The groundwater surface has fallen an average of • 1.83 feet since the last monitoring event. Groundwater 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 Methods 5030 and 8020 with DHS/LUFT Method. • The sample from well MW-1 was also analyzed for total oil and grease (TOG) using Standard Methods 5520C and 5520F. The samples were analyzed at Columbia Analytical Services, Inc. in San Jose, California (Hazardous Waste Testing Laboratory Certificate No. 1426). The results of these analyses are summarized in Table 1. Copies of the laboratory analysis reports are attached. 0 Results of analyses indicated TPHg and BTEX concentrations were below laboratory detection limits in groundwater samples collected from wells MW-2, MW-3, and MW-4. TPHg was detected in wells MW-1 and MW-5 at concentrations of 130 and 1,200 parts per billion (ppb), respectively, and benzene was detected in wells MW-1 and MW-5 at • concentrations of 0.9 and 76 ppb, respectively. TOG was not detected using either method . ioolcmr 30004-a 2 •