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_7 11FAMARAIR June 21, 1993 Working To Restore Nature . ARCO Service Station No. 0548, Stockton. California Results of analyses are shown on Table 1. TPHg and benzene were detected at 39,000 ppm and 660 ppm, respectively. GROUND-WATER MONITORING Subjective Analysis and Water-L&vel MeasureMents On March 4, 1992, a technician from Emcon measured depths to water, performed subjective analyses of ground water, and collected ground-water samples from wells MW-2 01. _ _ _ through MW-4 for laboratory analyses (MW-1 was inaccessible due to asphalt covering the wellhead). On March 26, 1993, a technician from RESNA measured the depth to water in well MW-5 and then developed the we11. Our technician also visited the site.on March 29, 1993, to measure the depth to water and collect.a ground-water sample from well MW-5 for laboratory analyses. No floating product or product sheen was noted in samples from the • wells during the site visits (Table 2). Data obtained from Emcon is presented in the Appendix to this report. Our field methods are attached. The wellhead elevations of the monitoring wells at the site were previously surveyed by Tom O. Morrow of West Sacramento, California, a licensed land surveyor (California Land 0 Surveyors No.5161). Well MW-5 was also surveyed by Tom O. Morrow on March 22, 1993; due to different gauging dates this data was not used to construct this potentiometric map. The surveyed elevations and the depth-to-groundwater measurements (Table 2) were used to evaluate the potentiometric surface beneath the site. The ground-water flow direction on March 4, 1993, was generally to the east with a hydraulic gradient of 0.003 (Plate 15). • Ground-Water Sampling and Analyses Ground-water samples were analyzed for TPHg and BTEX using EPA Methods 5030 and 8020 and standard methods prescribed by the State of California. These samples were analyzed at Columbia Analytical Services, Inc. in San Jose, California (Hazardous Waste Testing Laboratory Certificate No. 1426). The ground-water sample collected from well ` MW-5 was submitted to Sequoia for analysis of TOG using Standard Method 5520 E&F, TPHg, and BTEX. The results of analyses are summarized in Table 2. Copies of the laboratory analysis reports are in the Appendix. • The results of analyses indicated that TPHg was detected in samples collected from wells MW-2 and MW-5 at concentrations of 57 and 930 parts per billion (ppb), respectively. TPHg was below laboratory detection limits in the remaining wells sampled. Benzene was detected at 16 ppb in well MW-5 but was not detected in wells MW-2, MW-3, and MW-4. The laboratory report indicates that the sample collected from wells MW-2 contains lower boiling point hydrocarbons which do not match typical gasoline fingerprints. The report also • 033lccar 30004-8 3 •