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1 <br /> July 31, 1992 <br /> ARCO Service Station No 2130, Stockton, California <br /> "'oplangTo Restore Nature <br /> 1 The results of analyses indicated that _TPHg was detected in samples from the three wells <br /> at concentrations ranging from 110 to 5,200 parts per billion (ppb), and concentrations of <br /> benzene ranged from 4 9 to 1,400 ppb Fecal colif6rm bacteria were not detected in <br /> samples from the three wells <br />` WORK IN PROGRESS <br /> At the request of ARCO, we have prepared a,Work Plan Addendum for further soil and <br /> water assessment and an aquifer sparging pilot test at the site Additional vapor-extraction <br /> wells will be installed during the additional assessment, and the sparging pilot test will be <br />` performed to evaluate groundwater remediation <br /> CONCLUSIONS AND RECOMMENDATIONS <br /> The ground-water flow direction has consistently been to the southeast since January 1992 <br /> The depth to ground water dropped approximately 0 6 feet since the previous quarter <br /> TPHg was detected in water from the three wells at concentrations ranging from 110 to <br /> 5,200 ppb, and benzene was detected at concentrations above MCL's The TPHg and <br /> benzene concentrations detected in the wells during this quarterly sampling event are <br /> approximately 82 percent and 60 percent lower, respectively, than the concentrations <br /> . detected during the previous event In our opinion, continued quarterly monitoring is <br /> warranted We understand that Emcon will conduct the next quarterly monitoring event in <br /> June 1992 We recommend that copies of this report be forwarded to Mr Michael Infurna <br /> of the San Joaquin Public Health Services, P O Box 2009, Stockton, California 95201 and <br /> Mr James Mangham of the Central Valley Regional Water Quality Control Board, 3443 <br /> Routier Road, Sacramento, California 95827-3098 <br /> 303-6 <br /> 300003-6 3 <br />