Monitoring of MTBE Attenuation by Stable Isotope Analysis

Compound-specific isotope analysis (CSIA) is a technique gaining popularity in the studies of biodegradation of groundwater contaminants (such as BTEX, chlorinated ethenes and MTBE). The benefit of CSIA is in providing a contaminant-specific tool for monitoring the effects of in-situ biodegradation, eliminating the need to verify biodegradation potential of a site by microcosm experiments. This presentation will summarize the application of CSIA to MTBE attenuation studies. Results published to date and new experimental data indicate that: (i) it is practical to distinguish between the effects of aerobic and anaerobic MTBE biodegradation by combined carbon + hydrogen CSIA; (ii) the magnitude of carbon isotopic fractionation in the anaerobic process is large and apparently consistent among different anaerobic microbial cultures; and (iii) there is little evidence of anaerobic mineralization of the tert-butyl group of MTBE (tert-butyl alcohol accumulates upon MTBE degradation). The standard practice in interpreting CSIA data is to attribute the observed isotope effects to degradative processes. Published data on isotope effects upon phase partitioning, volatilization etc. suggest that these processes result with minor isotope fractionation and should not interfere with the studies of biodegradation. It will be shown that under certain environmental conditions, measurable changes of carbon and hydrogen isotope ratios are likely due to MTBE volatilization. While the magnitude of those changes is low in comparison with those due to anaerobic biodegradation, volatilization and aerobic biodegradation can be difficult to distinguish from each other. Evaluation of in-situ biodegradation for weakly fractionating compounds, in particular those with few sampling points and those utilizing only one isotope species (as opposed to 2-D CSIA) should take into account the potential for isotope effects caused by non-degradative attenuation. Adjustments to “traditional” protocols of CSIA data evaluation will be proposed to account for the isotope effects resulting from non-degradative processes.