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I NSA-�� October 14, 1993 Working To Restore Nature i ARCO Service Station No 2130, Stockton, California GROUNDWATER MONITORING: THIRD QUARTER 1993 On July 30, 1993, a technician from EMCON measured depths to water and collected groundwater samples from wells MW-1 through MW-7 for subjective and laboratory analyses. Floating product or product sheen was not noted in samples from the wells (Table 1) 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 Surveyors No 5161) The surveyed elevations and the depth-to-groundwater measurements, summarized in Table 1, were used to evaluate the potentiometric surface beneath the site The groundwater flow direction on July 30, 1993, was to the east-southeast with a hydraulic gradient of 0 001 (Plate 3) The groundwater flow direction in the past has generally been towards the southeast The groundwater elevation beneath the site decreased approximately 0 73 foot since the last sampling event Groundwater elevation data are presented on Plate 3 and in Table 1 Groundwater samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg) and benzene, toluene, ethylbenzene, and total xylene isomers (BTEX) using Environmental Protection Agency Methods 5030/LUFT and 5030/8020 These samples were analyzed at Columbia Analytical Services, Inc. in San Jose, California (Hazardous Waste Testing Laboratory Certificate No 1426) 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 dissolved gasoline hydrocarbons were detected in samples from wells MW-1 through MW-6 at concentrations ranging from 89 parts per billion (ppb) (well MW-3) to 2,100 ppb (well MW-1), concentrations of benzene in these wells ranged from less than detection limits of 0 5 (well MW-6) to 390 ppb (well MW-1) TPHg and BTEX were not detected in the samples collected from well MW-7 TPHg and benzene concentrations are shown on Plate 2 WORK IN PROGRESS The underground storage tanks (UST's) and product lines will be removed and replaced during September 1993 Installation of subgrade piping for future site remediation will be performed concurrently with the replacement of UST's and product lines RESNA is obtaining encroachment permits to install two offsite monitoring wells upgradient and cross gradient of the site. We anticipate that drilling operations will be conducted in October, 1993 It is our understanding that EMCON is scheduled to perform the next groundwater sampling event at the end of October 1993 08?Sccar 30003-9 2