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COMPOUND-SPECIFIC ISOTOPE ANALYSIS OF MTBE AND TBA FOR BIOREMEDIATION STUDIES
Citation:
Kuder, T., R. Kolhatkar, J T. Wilson*, P. Philp, AND J. Allen. COMPOUND-SPECIFIC ISOTOPE ANALYSIS OF MTBE AND TBA FOR BIOREMEDIATION STUDIES.
Impact/Purpose:
To inform the public.
Description:
The utility of stable isotope ratios to detect biodegradation for a number of chemical compounds including MTBE and TBA has been demonstrated in a number of laboratory or field studies. Chemical reactions tend to favor molecules with the lighter isotopic species (e.g., 12C, 1H), resulting in enrichment of the heavier isotopic species (13C, D) in the unreacted substrate, an effect referred to as isotopic fractionation. Recent developments in gas chromatography-isotope ratio mass spectrometry (GCIRMS) permit determination of the isotopic composition (carbon and hydrogen) of individual compounds at ppb concentrations, in groundwater collected from contaminated aquifers.
The current presentation will show examples of composite field-microcosm data sets, where in-situ degradation signatures were measured on groundwater samples and the microcosms provided reference data for field data interpretation. The laboratory data permit calculation of quantitative parameters of isotope fractionation (i.e., the enrichment factor of the reaction). Field data are evaluated using the frame of reference provided by stable isotopic data from microcosms to estimate the in-situ biodegradation extent. This approach permits a conservative quantitative determination of MTBE biodegradation as opposed to overall attenuation (biodegradation plus non-degradative attenuation). Work is underway to collect a representative database of microcosm data and define a range for the enrichment factor parameter, potentially allowing quantitative data interpretation without the necessity of collecting isotope data from microcosms developed from a specific field site.
The second topic of the presentation will be TBA biodegradation. In general GCIRMS analyses of TBA samples at sites studied to date have not provided an isotope signature for biodegradation. However, preliminary work has allowed identification of few sites where TBA isotope values suggest biodegradation.