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IS YOUR TBA COMING FROM BIODEGRADATION OF MTBE
Wilson*, J T. IS YOUR TBA COMING FROM BIODEGRADATION OF MTBE. Presented at 2004 LUST Nat'l. Conf, New Orleans, LA, March 01 - 03, 2004.
To inform the public.
MTBE (methyl tertiary butyl ether) is present at high concentrations in ground water at many sites where gasoline has been spilled from underground storage tanks. In addition, TBA (tertiary butyl alcohol) is also present at high concentrations in many of the same ground waters. TBA is regulated as a contaminant by several of the states, including the state of California. In 2002 in Orange County, California there were 429 gasoline spill sites that reported concentrations of both MTBE and TBA. The geometric mean of MTBE concentrations was 627 ug/liter. The geometric mean of TBA concentrations was 893 ug/liter. To manage the hazard associated with the presence of TBA in groundwater, staff of the UST Local Oversight Program (LOP) of the Orange County Health Care Agency needed to know the source of the TBA. There are several plausible sources: TBA has been used as an oxygenate in gasoline in some parts of the US, it is a trace component of technical MTBE used in gasoline, and it can be produced by biological degradation of MTBE to TBA.
When microorganisms degrade MTBE to TBA, they discriminate between molecules of MTBE containing the stable isotope 13C and molecules containing 12C. Molecules containing 12C are preferred and molecules containing 13C accumulate in the residual MTBE that has not been degraded. The extent of biodegradation of MTBE to TBA in ground water can be inferred from the shift in the ratio of 13C to 12C in the residual MTBE. From the gasoline spills in Orange County, 13 sites were selected for a detailed SCIR study. One gasoline spill in Delaware was also included. Some of the sites had high concentrations of both TBA and MTBE, while other sites had high concentrations of TBA but much lower concentrations of MTBE. At most of the sites concentrations of MTBE went down over time while concentrations of TBA increased. The value of the Stable Carbon Isotope Ratio in residual MTBE in ground water at the sites was used to predict the extent of degradation of MTBE (C/Co). The extent of degradation was used to predict the concentration of TBA in ground water according to the three methods mentioned below. For each method, the predicted concentration of TBA was compared to the measured concentration.
Method (1) = (Conc. MTBE in the Current Sample / (C/Co))*(74/88)
Method (2) = Maximum Historical Conc. MTBE in a well * (1-C/Co)* (74/88)
Method (3) = Maximum Historical Conc. MTBE on site * (1-C/Co)* (74/88)
Natural anaerobic biodegradation was accepted as an explanation for the concentrations of TBA if the predicted concentrations accounted for most of the TBA actually measured (i.e., the concentration of TBA predicted > 0.5 of the concentration of TBA actually measured).
A total of 14 sites were studied in detail. At 12 of the sites natural anaerobic biodegradation of MTBE was the most plausible explanation for the high concentrations of TBA in the ground water. The analysis of the Stable Carbon Isotope Ratio in residual MTBE was a useful tool to identify sites where TBA in ground water was produced by natural anaerobic biodegradation of MTBE.