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AMAF AM January 11, 1992 Working To Restore Nature •'� ARCO Station No. 548, Stockton, California analytically identified in ground-water 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 September 4, 1992, to take water-level measurements and collect ground-water samples from monitoring wells MW-1 through MW- 4 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-ground-water measurements (summarized in Table 1) were used to evaluate the potentiometric surface beneath the site. • On September 4, 1992, the direction of ground-water flow was southeast with a gradient of approximately 0.005 (Plate 3). The direction of ground-water flow has generally been southeast since our investigation began. The ground-water surface has fallen an average of 1.74 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 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-2, MW-3, and MW-4. TPHg and benzene were detected at concentrations of 260 and 8.7 parts per billion (ppb), respectively, in the ground-water sample collected from well MW-1. • WORK IN PROGRESS RESNA is currently preparing 1) a work plan for installing and operating a soil remediation system at the site, 2) design and construction specifications and 3) necessary permit • packages. The system will be installed after the work plan and permits have been approved by the appropriate agencies and after ARCO selects a contractor for construction of the system. • • 1030ecar 30004-6 2