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WE <br /> CHEVRON RESEARCH AND TECHNOLOGY COMPANY <br /> Richmond, California <br /> February 05, 1991 <br /> Evaluation of Chromatograms <br /> from Chevron Service Station <br /> n 9-6171, Stockton <br /> Mr. C. B. Rogers: <br /> At your request I have evaluated Superior Analytical Lab's <br /> chromatograms of service station # 9-6171 fourth quarter 1990 water <br /> samples, and have the following observations. <br /> The 159 ppb TPH-gascline result for sample 11254-4 MW-4 appears to <br /> be valid, but the hydrocarbon that was detected is not gasoline. It <br /> is instead two pure compounds, probably solvents. I cannot tell <br /> from the data what the compounds are. Pure speculation would <br /> suggest the two GCMS hits for MW-5 (tetrachloroethene and 1,2- <br /> dichloroethane) be considered as potential candidates for the two <br /> compounds in MW-4. You would need to analyze the MW-4 water by <br /> GC/MS to find out. <br /> The bailer blank 11254-2 BB result of 0.5 ug/L toluene appears to <br /> be caused by rounding off the result to one significant figure <br /> (sf) . If you calculate the concentration of toluene in water b-.- <br /> dividing <br /> •-dividing the reported 2.4 ng toluene (2 sf) by 5.0 ml (2 sf) and <br /> converting to ug/L, you get a concentration of 0.48 ug/L (2 sf) . If <br /> the detection limit for toluene is 0.5 ug/L, the same as that <br /> reported for all the rest of the compounds, then the result should <br /> he reported as <0.5 ug/I. There may be other round-off errors <br /> associated with the calculation of the amount (ng) toluene purged. <br /> superior's multi-point calibration data is neede,; in order to check <br /> this. If you wish to pursue this issue, ask Superior to recalculate <br /> without rounding off intermediate results. <br /> The GC/MS results reporting tetrachloroethylene (perchloroethylene, <br /> PCE) and 1,2-dichloroethane appear to be valid. Tetrachloroethylene <br /> is not a component of gasoline but is commonly used as a degreaser <br /> at service stations and as dry cleaning fluid. 1,2-dichloroethane <br /> is added to leaded gaso7.ines to'prevent lead deposits in automobile <br /> exhaust manifolds. <br /> If I can answer any more questions, please call me at CTN 620-4993. <br /> E. A. Harvey <br /> ,CtRogers <br /> AigVerstuyft <br /> �f EAHarvey <br /> file <br />