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e� <br /> R101N <br /> Memorandum <br /> To: Debra Hamlin, Bridgestone/Firestone <br /> From: Harry Takach <br /> Copy: <br /> Date: 3/5/01 <br /> Subject: Revised TPH Mass Calculation for Stockton Property <br /> The San Joaquin County Environmental Health Division and the California Regional <br /> Water Quality Control Board have requested that Bridgestone/ Firestone (BFS) revise the <br /> submitted mass calculation for total petroleum hydrocarbons (TPH) at the 400 North EI <br /> Dorado Avenue site in Stockton, California. At the request of BFS, Orion Environmental <br /> Inc. (Orion) has reviewed the file for additional soil and groundwater TPH concentrations <br /> from previous consultants' reports for this site. <br /> Background - The agency concern is focused on one data point collected from an angle <br /> boring, P13-1, performed by Foothill Engineering on 10 April 1997. From soil samples <br /> collected at 5-foot intervals, the sample at 20 feet (approximately 18 feet below surface <br /> and below Tank No. 3) showed a TPH concentration of 16,200 mg/kg. Samples directly <br /> above and below this point were non-detect and 108 mg/kg, respectively. A second <br /> probe, PB-2, approximately 15 feet to the northwest had a TPH concentration at 20 foot <br /> depth of 1,160 mg/kg and non-detect above and below this sample. QST Environmental <br /> sampled soils from two auger borings in 1998 and did not detect TPH in four samples <br /> from the two borings located approximately 20 feet away from the original PB-1 location. <br /> During the installation of three groundwater monitoring wells by Orion in January 2000, <br /> one soil sample from each boring was analyzed and reported as non-detect for TPH. <br /> Mass Calculation The historical soil sample data indicate that the volume of impacted <br /> soil is limited to the material found just below the closed-in-place Tank No. 3. The depth <br /> to water from the groundwater monitoring wells nearby is 19 to 20 feet below grade. The <br /> tank bottom was estimated at approximately 12 feet below the surface. For this <br /> calculation, Orion assumed that the impacted soil volume is a 10-foot-diameter circle <br /> with a vertical height of 5 feet. Therefore, the estimated volume is 392.7 cubic feet. <br /> Since the concentration goes from the 16,200 mg/kg TPH to non-detect rapidly in both <br /> vertical and horizontal directions, a conservative assumption is an average of 6,000 mg/kg <br /> of TPH within this impacted volume. Using this basis, the mass of hydrocarbon at the site <br /> is estimated to be 2.59 pounds or 38 gallons of weathered gasoline. <br /> Z1 -F4 <br /> -mss <br /> [S:BFSIStockton\Mem0\MEM0002j 3/5/01 <br />