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' August 18, 1992 Working To Restore Nature ARCO Station No. 548, Stockton, California California. Dissolved gasoline-hydrocarbons. have been_analytically-,identified_.in_grounu_ wa#er samples collected-from monitoring well MW-1. Dissolved gasoline hydrocarbons were not detected�in ground-water samples collected from the remaining wells (RESNA,June 26, 1992, Report No. 30004-6). GROUND-WATER MONITORING A technician from Emcon visited the site on June 10, 1992, to take water-level measurements, perform subjective analyses of ground water, and collect ground-water • samples from monitoring wells MW-1 through MW-4 for 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-ground-water measurements (summarized in Table 1) were used to evaluate the potentiometric surface beneath the site. On June 10, 1992, the direction of ground-water flow was southeast with a.gradient of approximately 0.004 -(Plate 3). - The direction of ground-water_ flow has_generally been southeast.since our investigation.began. The groundwater surface has fallen an average of 00 1.70 feet since the last 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 these analyses are summarized in Tables 2. 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-2 and MW-4. Benzene • t was detected at a concentration.of 1.0 part per biiliori in the_ground-water sample collected -. _w_ -J from_well.MW-1; TPHg, toluene, ethylbenzene, and total xylene isomers were not detected in this_sampie._ TPHg_wasdetected in-the-water sample collected from well-MW-3 at a concentration of 60.ppb,_however no other_analytes were detected."_� CONCLUSIONS AND RECOMMENDATIONS The ground-water flow direction has consistently been southeast since October 1991. Low concentrations of hydrocarbons were detected in ground-water samples collected from monitoring wells (MW-1 and MW-3). Dissolved hydrocarbons were not detected in monitoring wells MW-2 and MW-4. r070&car 30004-6 2