You are here:
APPLICATION OF STABLE CARBON AND HYDROGEN ISOTOPIC TECHNIQUES FOR MONITORING BIODEGRADATION OF MTBE IN THE FIELD
Citation:
Kuder, T., P. Philp, R. Kolhatkar, J T. Wilson*, AND J. Allen. APPLICATION OF STABLE CARBON AND HYDROGEN ISOTOPIC TECHNIQUES FOR MONITORING BIODEGRADATION OF MTBE IN THE FIELD. 2002 Petroleum Hydrocarbons Conference & Exposition #5040, Atlanta, GA, 11/5-8/2002.
Description:
A significant challenge in environmental studies is to determine the onset and extent of MTBE bioremediation at an affected site, which may involve indirect approaches such as microcosm verification of microbial activities at a given site. Stable isotopic fractionation is characteristic of biodegradation processes of organic compounds and may be potentially applied in MTBE studies. Isotopic composition (expressed as d13C or dD in units of per mil) is a measure of the ratio of concentrations of stable isotopes of carbon (13C/12C) and hydrogen (D/H). It is known that biodegradation of low molecular weight compounds preferentially removes the isotopically lighter species (12C or 1H), resulting in enrichment of 13C or D in the remaining substrate (isotopic fractionation). Both carbon and hydrogen isotope fractionation has been described by a number of authors in particular for BTEX and chlorinated ethenes in microcosms and in the field and recently for MTBE in lab biodegradation experiments. The extent of isotopic fractionation may be used as a proxy for the extent of degradation. Since isotopic fractionation can be mathematically modeled using the Rayleigh equation, prediction (extrapolation) and back-extrapolation of concentration/isotopic composition data may be feasible. Gas chromatography-isotope ratio mass spectrometry (GCIRMS), combined with purge and trap (PT), solid phase microextraction (SPME), or other relevant methods, permits analyses of environmental samples to determine the isotopic composition of individual components. GCIRMS allows very low quantitation limits and at present our methods permit analysis of MTBE at sub-ppb level in groundwater.
This presentation shows results from a number of field sites affected by gasoline spills. Samples of groundwater and microcosm cultures from two gasoline-range hydrocarbon plumes at gas stations sites in New Jersey and a microcosm experiment containing indigenous bacteria from an MTBE spill site in Texas have been characterized. The sites were originally anoxic (methanogenic). One of the NJ sites had oxygen infusion wells installed and redox status downgradient of well was oxic. d13C and dD of the MTBE and TBA was determined for all samples wherever possible by GC-IRMS. All three data sets were similar in several aspects. TBA was a major compound in all of the samples where MTBE showed isotopic fractionation, while it was absent, or relatively insignificant, in control samples or samples containing isotopically lighter MTBE. There was a very good match of all microcosm and field samples to a Rayleigh-type fractionation model. Interestingly, the observed fractionations were significantly larger than in the microcosm data published elsewhere. With a similar degree of substrate depletion, d13C offset was one order of magnitude higher (loss of ca. 95% MTBE resulted in d13C effect in excess of 40 per mil). The three data sets were similar in the extent of fractionation, notwithstanding the different biogeochemical regimes. dD fractionation was similar in direction and extent as for carbon and also similar between the aerobic and anaerobic sites. The results are discussed in context of potential biochemical differences between MTBE-degrading organisms and strategies of biodegradation identification in the field.