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r� � � AWAWuAM November 1, 1993 ZMAWS a ARCO Service Station 2168, Stockton, California Working to Restore Nature In January 1993, RESNA observed drilling of seven soil borings and the installation of two groundwater monitoring wells and four vapor extraction wells • GROUNDWATER MONITORING: THIRD QUARTER 1993 EMCON of Sacramento, California, was contracted by ARCO to conduct the field work at the site, EMCON subsequently forwarded their field notes and the laboratory reports to RESNA Field work included measuring depths to water and collecting groundwater w samples for subjective and laboratory analyses RESNA used these data to evaluate the direction and gradient of groundwater flow beneath the site The locations of the monitoring wells are shown on Plate 2 On September 2, 1993, a technician from EMCON measured depths to water and collected groundwater samples from the wells for subjective and laboratory analyses The technician noted no floating product or sheen in the five wells (Table 1) The wellhead elevations of the monitoring wells at the site were previously surveyed by Tom O Morrow, Inc of West Sacramento California, a licensed land surveyor (California Land Surveyors No 3275) The surveyed elevations and the depth-to-groundwater measurements, summarized in Table 1, were used to evaluate the potentiometric surface beneath the site The direction of groundwater flow on September 2, 1993 was northeast with a hydraulic gradient of 0 003 as shown on Plate 3 The groundwater elevation decreased by approximately 0 89 foot since the previous monitoring event Groundwater samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg) and benzene, toluene, ethylbenzene, and total xylene isomers (BTEX) using Environmental Protection Agency (EPA) Methods 5030 and 8020 and DHS LUFT methods prescribed by the State of California Samples were also analyzed for halogenated volatile organic compounds using EPA Methods 5030/601, and the sample from well MW-2 was analyzed for TOG using Standard Methods (SM) 5520C and 552OF 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 Tables 1 and 2 Copies of the laboratory analysis reports are in the appendix Results of analyses indicated TPHg and BTEX were detected in samples from the five wells Concentrations of TPHg ranged from 18,000 parts per billion (ppb) to 110,000 ppb, and benzene concentrations ranged from 3,300 ppb to 13,000 ppb 1,2-Di ch I oro ethane was detected in groundwater samples from the wells at concentrations ranging from 0 6 ppb to 200 ppb, 1,2-dichioropropane was detected in wells MW-2, MW-4, and MW-5 at concentrations of 4 5 ppb, 12 ppb, and 7 5 ppb, respectively TOG was detected in well MW-2 at concentrations of 10 ppb and 9 0 ppb using SM 5520C and 5520F, respectively I004ccar 31009-9 2