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COMPOUND-SPECIFIC STABLE ISOTOPE ANALYSIS TO DEMONSTRATE IN-SITU MTBE BIOTRANSFORMATION
Citation:
Kuder, T., R. Kolhatkar, J T. Wilson*, AND P. Philp. COMPOUND-SPECIFIC STABLE ISOTOPE ANALYSIS TO DEMONSTRATE IN-SITU MTBE BIOTRANSFORMATION. Presented at Seventh International Symposium Battelle Conference, Orlando, FL, June 02 - 05, 2002.
Impact/Purpose:
To inform the public.
Description:
Change of stable isotope composition of organic contaminants (isotopic fractionation) is a useful indicator of biotransformation. Most of applications to date are in the area of chlorinated solvents and recently BTEX, MTBE and TBA. Chemical reactions (biotic- and abiotic transformation) tend to favor molecules with the lighter isotopic species (e.g., 12C, 1H), resulting in enrichment of the unreacted substrate in the heavier isotopic species (13C, D), while the reaction products become enriched in the lighter isotopes. Although fractionation may be caused by a multitude of other processes, such as dissolution, volatilization, sorption etc., the potential magnitude of these effects is minimal and does not limit the practical utility of the approach. Development of gas chromatography-isotope ratio mass spectrometry allowed to study isotopic composition of single compounds in environmental samples by incorporation of standard preparative techniques, such as purge and trap. The sensitivities achieved to date are fully satisfactory for environmental purposes (lower ppb to sub-ppb levels). The technique will supplement classic biogeochemical and/or microbiological indicators. Stable isotopes offer an interesting alternative to time-consuming microcosm experiments as third line of evidence to demonstrate natural biodegradation of MTBE, the standard approach to verify biodegradation at a given site.
The data on MTBE presented here and previous results by ours and other groups confirm fractionation during biodegradation of MTBE, which can be relatively easily measured in ground water samples. The focus of this presentation is on showing variable pathways of isotopic fractionation at different sites. The practical significance of the isotope data would be greatly improved by better understanding of the isotopic expression of degradative reactions, by allowing quantitative evaluation of the extent and rates of remediation. Currently, the preliminary data suggest that the extent of C and H fractionation is strikingly different among MTBE plumes (and microcosm studies). The plumes may be broadly divided into: 1) plumes where major effects are observed for both C and H (tens of per mil of d13C and dD); 2) plumes where carbon fractionation is small (not exceeding few per mil d13C), but the hydrogen effect is an order or two of magnitude more intense than carbon effect (up to over a hundred per mil of dD). The former case was verified by microcosm study of anaerobic cultures collected from our field site (carbon isotopes only). The latter case is consistent with aerobic microcosm studies published by other groups. Importantly, a number of field sites, which were selected for isotope study as likely to be affected by biodegradation, did not yield positive results. Work is in progress to determine whether biodegradation is possible with no isotopic effect (thus false negatives may be generated) or that other factors were involved (e.g., concentration decrease by dispersion or sample coverage insufficient, missing the active parts of a plume). Data sets representing both situations are shown and discussed as examples for field data evaluation.