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• BP OIL i January 12, 1998 San Joaquin County Public Health Services Attention Mr. Harlin Knoll - Environmental Health Division P.O. Box 200 Stockton, A 95201 RE: Former BP Oil Site No. 11194 2375 Tracy Boulevard Tracy, CA Dear Mr. Knoll: • :PP Oil Company Environmental Remediation Management 29kSW 41st Street lk4don, Washington 98055-4931 (425) 251-0667 Fax No: (425) 251-0736 Enclosed find Groundwater Monitoring and Sampling Report, dated 5 December 1997. Mobil Oil Corporation reported a petroleum release at this site during 1986 when the unleaded underground storage tank system failed to pass a tightness test. The steel tanks, believed to have been installed in 1967, were consecluentially replaced with , double -walled fiberglass s s 6. When the fiberglass s were installed in the excavation dug to remove the steel tanks, a sheen was observed on water in the excavation. Mobil performed a site assessment and groundwater monitoring activities after the tanks were replaced, and later transferred management of the cleanup to BP after BP purchased the site in 1989. BP subsequently sold the site to the current operator (Tosco Corporation) during 1994 and continues to monitor the groundwater. It is believed that the current tanks did not fail any required tightness tests when BP operated the site. December 1998 regulations for leak detection and prevention are understood to require a number of improvements to current system, including the installation of overfill protection, overspill protection, and containment pans beneath the product dispensers. The enclosed report summarizes chemical data collected since 1986. Note that some of the previous reports omitted data obtained prior to 1990 and this data will be included in future groundwater monitoring reports. Measurable liquid petroleum hydrocarbon accumulations -- previously observed in wells MW -3, MW -4, and MW -6 -- have not been observed since October, 1996. It is noted that the aromatic hydrocarbon concentrations detected in wells where liquid petroleum hydrocarbons (LPH) have been observed are very low compared to what would be expected when unweathered product is released into groundwater. For example, when gasoline containing 1.20 volume percent benzene' is mixed with water, an equilibrium benzene water concentration of ' It is noted that 1.20 volume percent benzene represents the Cap limit for "Cleaner -Burning Gasoline" required by CARB since spring, 1996. Gasoline sold in California prior to that time often contained more