| Abstract
| - Compound-specific analysis of stable carbon and hydrogenisotopes was used to assess the fate of the gasolineadditive methyl tert-butyl ether (MTBE) and its majordegradation product tert-butyl alcohol (TBA) in a groundwaterplume at an industrial disposal site. We present a novelapproach to evaluate two-dimensional compound-specificisotope data with the potential to identify reactionmechanisms and to quantify the extent of biodegradationat complex field sites. Due to the widespread contaminantplume, multiple MTBE sources, the presence of numerousother organic pollutants, and the complex biogeochemicaland hydrological regime at the site, a traditional mass balanceapproach was not applicable. The isotopic compositionof MTBE steadily changed from the source regions alongthe major contaminant plume (−26.4‰ to +40.0‰(carbon); −73.1‰ to +60.3‰ (hydrogen)) indicatingsubstantial biodegradation. Constant carbon isotopicsignatures of TBA suggest the absence of TBA degradationat the site. Published carbon and hydrogen isotopefractionation data for biodegradation of MTBE under oxicand anoxic conditions, respectively, were examined andused to determine both the nature and the extent of in-situ biodegradation along the plume(s). The coupledevaluation of two-dimensional compound-specific isotopedata explained both carbon and hydrogen fractionationdata in a consistent way and indicate anaerobic biodegradation of MTBE along the entire plume. A novel scheme toreevaluate empiric isotopic enrichment factors (ε) in termsof theoretically based intrinsic carbon (12k/13k) andhydrogen (1k/2k) kinetic isotope effects (KIE) is presented.Carbon and hydrogen KIE values, calculated for differentpotential reaction mechanisms, imply that anaerobicbiodegradation of MTBE follows a SN2-type reactionmechanism. Furthermore, our data suggest that additionalremoval process(es) such as evaporation contributed tothe overall MTBE removal along the plume, a phenomenonthat might be significant also for other field sites attropic or subtropic climates with elevated groundwatertemperatures (25°C).
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