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REFAWAWAkMAM Working To Restore Nature September 13, 1993 •� ARCO Station No. 548, Stockton, California On April 10, 1993, a product line was accidently fractured and leaked product into adjacent soil. RESNA collected a soil sample from 5 feet below the product line; gasoline hydrocarbons were detected in this sample. • GROUNDWATER MONITORING On June 7, 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 June 7, 1992, the direction of groundwater flow was east with a gradient of approximately 0.004 (Plate 3). The direction of groundwater flow has generally been southeast since our investigation began. The groundwater surface has risen an average of 1.46 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 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 sample from well MW-1 was also analyzed for total oil and grease (TOG) using Standard Method 5520B. The results of these 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 groundwater samples collected from wells MW-2, MW-3, and MW-4. TPHg was detected in wells MW-1 and MW-5 concentrations of 160 and 1,100 parts per billion (ppb), respectively, and benzene was detected in wells MW-1 and MW-5 at concentrations of 0.9 and 40 ppb, respectively. TOG was not detected in the sample from well MW-1. Concentrations of benzene and TPHg are shown on Plate 2. WORK IN PROGRESS RESNA is currently preparing a work plan to install a vapor-extraction well adjacent to the area where the unauthorized leak occurred. This well will be incorporated into the vapor- extraction system which will be installed at the site in the future. • 3OW4-8 3000a-a 2 •