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f <br /> May 6, 1992 <br /> Former Unocal Station 2859, Stockton, California <br />• <br /> Results of analyses of water samples indicated TPHg was detected at a concentration of <br /> 6,400 parts per billion (ppb) in water collected from onsite well MW-6 TPHg was detected <br /> at a concentration of 120 ppb in the sample from well MW-4, located on the east side of <br /> Pacific Avenue, however, water sampled from well MW-5 (north of well MW-4) did not <br />• contain detectable concentrations of TPHg or BTEX TPHg was detected at 98 ppb in well <br /> MW-7, installed further downgradient <br /> Bailer blanks of distilled water collected from wells MW-4 and MW-7 immediately before <br /> collecting water samples were analyzed for quality assurance Dissolved hydrocarbons were <br /> not detected in these water samples, results are designated with a "B" on Table 3 <br /> Evaluation of Ground-Water Gradient <br /> The seven monitoring wells were surveyed relative to mean sea level as described in the <br /> attached Field Procedures The ground-water gradient on March 4, 1992, was 0 0027, with <br /> a flow direction to the east, as shown on the Potentiometric Map (Plate 17) The ground- <br /> water flow direction and gradient are consistent with data collected before May 1990 The <br /> depth to water in each well is shown on Table 4 The ground-water surface rose <br /> approximately 14 feet since the November 1991 monitoring event <br /> 0* VAPOR-EXTRACTION TEST <br /> RESNA performed a VET onsite on March 16, 1992. We are currently evaluating the data <br /> collected during the test Results of the VET will be discussed in a later report <br /> • CONCLUSIONS <br /> Analytical results of the February 1992 drilling indicated residual hydrocarbons in soil are <br /> delineated Dissolved hydrocarbons were not delineated in ground water near the southern <br /> boundary of the site and offsite, therefore, submittal of the Problem Assessment Report will <br />• be deferred <br /> WORK IN PROGRESS <br /> The second quarter monitoring event is scheduled for May 1992 During the second quarter <br />* 1992, the VET report will be issued to Unocal To evaluate whether other possible sources <br /> of gasoline hydrocarbons are present south or east of the site, we recommend examining <br /> historic air photos of the area and searching county and state records for known or <br /> suspected releases near the site After completing these tasks, we will submit a work plan <br /> addendum to the San Joaquin County Public Health Services, Environmental Health <br />• Division (SJCPHS-EHD) to complete delineation of dissolved hydrocarbons <br /> 0317bbou <br /> 38012-9 4 <br /> 0 <br />