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MEMORANDUM Cal/EPA ♦ •p 'a o CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD CENTRAL VALLEY REGION 4�r<e ow.�*'�c� (916)255-3000 - CALNET 8-494-3000 - FAX(916)255-3015 Pete Wilson,Governor TO: Gordon L. Boggs FROM: Patricia A. Anderson DATE: 14 January 1998 SIGNATURE: SUBJECT: REVIEW FOR CLOSURE,BP OIL STATION NO. 11191, 1469 E. HAMMER LANE, STOCKTON, SAN JOA QUIN COUNTY Based on my review of the case file concerning the above site, I have noted the following: Background: In 1989,this site, owned by BP Oil Co., had a 12,000 and a 10,000 gallon unleaded gasoline tanks, a 6,000-gallon regular gasoline tank and a 6,000 diesel tank. The 6,000-gallon diesel and regular gasoline tanks, failing precision tests in 1988, 1989, and 1990,were repaired. These underground storage tanks (USTs) were removed by BP and replaced with three 10,000-gallon gasoline tanks and a 10,000-gallon diesel tank. However,the treatment and disposal of the residual petroleum product, and the disposal of the tanks and associated piping is unknown. Because a significant release of product occurred from the regular gasoline tank in 1989, five soil borings were drilled in 1992 and 1993. Petroleum contaminants were detected in soil samples from these borings at 46.5 and 51.5 feet below ground surface (ft bgs) near the northwest dispenser. The samples had 0.15 and 0.0093 mg/kg benzene (B), 0.13 mg/kg toluene (T), 0.014 mg/kg ethylbenzene (E), and 0.04 mg/kg xylene (X). In November 1994, additional soil samples were collected to determine the extent of soil contamination. Samples beneath the dispenser had 3,400 and 40,000 mg/kg total petroleum hydrocarbons as diesel (TPH-d), 28 and 29 mg/kg TPH as gasoline (TPH-g), and 0.5 mg/kg xylene. Soil samples from the stockpiled contaminated soil had 5.3 mg/kg total petroleum hydrocarbons as gasoline (TPH-g), 0.037 mg/kg ethylbenzene, and 0.25 mg/kg xylene. The amount and final disposal of this soil is unknown. Ground water was not encountered in the borings. The San Joaquin County case closure summary states the depth to water ranges from 50 to 80 ft bgs. _ Conclusions: The hydrocarbon contamination beneath the northwest dispenser extends laterally at least 20 ft and vertically at least 51.5 ft bgs. The vertical extent of contamination was not determined beneath the dispenser, where the highest concentration of contamination was detected. The soil contamination could be impacting ground water at its present known depth. Tosco, the present owner of the site, submitted a workplan for replacing the current underground storage tanks and associated product lines in January 1998. They have proposed further investigation, if contaminated soil is encountered beneath the current tanks and pipelines. Recommendations: Board staff have determined that closure of this site should be postponed until the investigation and, if necessary, remedial action by Tosco is completed.