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TBA IN GROUND WATER FROM THE NATURAL BIODEGRADATION OF MTBE
Wilson*, J T., P. Philip, AND T. Kuder. TBA IN GROUND WATER FROM THE NATURAL BIODEGRADATION OF MTBE. Presented at 15th Annual UST/LUST National Conference, San Francisco, CA, March 09 - 12, 2003.
To inform the public.
At many UST spills, the concentrations of TBA in ground water are much higher than would be expected from the presence of TBA in the gasoline originally spilled. The ratio of concentrations of TBA to concentrations of MTBE in monitoring wells at gasoline spill sites was compared to projections of the ratio that would be expected if the TBA in the ground water came from TBA that was originally present in the gasoline at a concentration of 10% of the concentration of MTBE, and if two-thirds of the MTBE remained with the gasoline and did not partition to ground water. The concentrations of TBA in ground water were higher than would be expected from the presence of TBA in the gasoline originally spilled in 50 of 74 stations surveyed in the Eastern United States, and in 319 of 429 stations in Orange County, California. At many gasoline spill sites, the concentrations of MTBE in monitoring wells attenuate over time, but the MTBE is replaced by TBA. This would be expected if attenuation in MTBE was caused by biological degradation to TBA. In example data from a gasoline station in Orange County, California, the concentration of MTBE in a monitoring well down gradient of the source of the spill decreased from 100 mg/L to 1 mg/L. The concentration of TBA stayed above 50 mg/L. Stable Isotope Ratio Analysis can be used to determine the important sources of TBA in UST monitoring wells. The stable carbon isotope ratio (SCIR, expressed as δ13C ) is the difference between the ratio of C13 to C12 in the sample, and the ratio in a standard, expressed in parts per thousand (‰). The MTBE originally present in gasoline is in the range of δ13C of -28‰ to δ13C of -33‰. If TBA is produced by biodegradation of MTBE, the carbon in the MTBE residual that is not degraded becomes enriched in the heavy isotope of carbon. The SCIR becomes larger (less negative), an enrichment of +5‰ is considered significant. Data were collected from five gasoline spill sites with high concentrations of TBA: a site in Parsippany, NJ and four sites in Orange County, CA. The attenuation of MTBE (C/Co) is estimated as the molar concentration of MTBE divided by the sum of the molar concentrations of TBA and MTBE. In all five stations, wells showed enrichment to a SCIR of -14‰ or lighter when the attenuation of MTBE exceeded 99% of the initial concentration. Anaerobic biodegradation of MTBE to TBA occurred at all of the five stations that had unexpectedly high concentrations of TBA.