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• <br /> Background <br /> 1 <br /> The Tesoro Refining, Marketing and Supply Company (Tesoro) <br /> conducted a systematic source investigation at its facility <br /> following the discovery on February 25, 1988, of significant <br /> gasoline leakage north of the Tesoro site along the Southern <br /> ePacific pipeline right-of-way and Time Oil property boundary. <br /> The purpose of the investigation was to determine if the gasoline <br /> leakage could be attributable to a source on the Tesoro site. <br /> On February 27, 1988, Tesoro voluntarily shut down its <br /> facility because of concerns that the source of the gasoline <br /> ' leakage had not been identified. As a first measure, Tesoro <br /> ' personnel conducted tank inventories to determine if the leakage <br /> potentially could have emanated from the gasoline storage <br /> ' tanks. No movement was detected in the three inventories taken <br /> that day or in any of the subsequent twice daily inventories <br /> taken through March 5, 1988 . Tesoro also contracted with Pelagos <br /> ' Corporation to perform pressure and accoustical testing on the <br /> eight-inch product delivery line from Southern Pacific and the <br /> ' three six-inch underground pipelines for transferring gasoline <br /> products from Tesoro' s tank farm to the loading rack. The <br /> ' testing indicated that all lines were tight (Appendix A) . <br /> ' To ensure that all potential sources of contamination were <br /> evaluated, Tesoro undertook field investigations to evaluate <br /> ' potential sources of the gasoline leakage. The investigation <br /> consisted of installing test holes at key points on the site; <br /> ' installing two monitoring wells; preparing an analytical program; <br /> ' - 2 - <br />