Bruce E. Rittmann, Environmental Engineering Program, Dept. of Civil Engineering, Northwestern University

In August 2000, the National Academy Press released Natural Attenuation for Groundwater Remediation, the report of a National Research Council (NRC) committee asked to provide expert guidance on when and how natural attenuation should be applied to remediate groundwater contamination. The author was the chair of the committee. Chapter 3 reviewed the scientific basis for natural attenuation of MTBE. It stated that MTBE is generally resistant to biodegradation, but that recent evidence shows that MTBE can be biodegraded co-metabolically by bacteria possessing certain oxygenase enzymes. At the time that the report was completed, the evidence suggested that MTBE biodegradation was possible, but that biodegradation may be slow or incomplete. Therefore, Chapter 3 concluded that the likelihood for successful natural attenuation of MTBE is low, given the current state of knowledge. Chapter 4 of Natural Attenuation for Groundwater Remediation outlined the necessary steps for evaluating whether or not natural attenuation is working and can be accepted at a site. The cornerstone of the evaluation is establishing a cause-and-effect relationship between loss of the contaminant and the natural-attenuation process responsible for that loss. Key to implementing the cause-and-effect criterion is the practical concept of "footprints," which are the observable consumptions or productions of other materials that participate stoichiometrically in the responsible natural-attenuation process. This presentation will evaluate more recent information on MTBE biodegradation in the light of the conceptual frameworks laid out by chapters 3 and 4. In particular, the presentation will tell whether or not the committee's conclusion of a low likelihood for success still holds. Furthermore, it will describe the key footprints needed to establish cause-and-effect for MTBE biodegradation as part of natural attenuation.

Kevin T. Finneran and Derek. R. Lovley, University of Massachusetts, Amherst

The potential for anaerobic degradation of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) was investigated. Laboratory incubations with a variety of contaminated sediments were screened for the capacity to degrade these compounds. MTBE degradation was stimulated in aquifer sediment that had been amended with Fe (III) oxides and humic substances. Humic substances and other extracellular quinones act as a soluble electron shuttle between the microorganisms and insoluble Fe (III) oxides, increasing Fe (III)-reducing activity. In some of the sediments amended with the electron shuttles and Fe (III) oxide, the rates of anaerobic MTBE oxidation were as rapid as the MTBE oxidation previously reported in aerobic sediments. Aquatic sediment that had been adapted to degrade MTBE converted [¹⁴C]-MTBE to ¹⁴CO₂ and ¹⁴CH₄ in the absence of any amendments. [¹⁴C]-MTBE was also mineralized in aquatic sediment adapted to degrade MTBE amended with Fe (III) oxides alone, and Fe (III) oxides plus the electron shuttling compounds. Chelated Fe (III) did not stimulate MTBE mineralization. Preliminary evidence with nitrate amended aquifer sediment indicates that nitrate may increase the extent of mineralization of [¹⁴C]-MTBE to ¹⁴CO₂ compared to unamended controls. The aquatic sediments also rapidly consumed TBA under anaerobic in situ conditions and converted [¹⁴C]-TBA to ¹⁴CH₄ and ¹⁴CO₂. Sediments did not require an adaptation period prior to the onset of TBA degradation. These results demonstrate that, under the appropriate conditions, MTBE and TBA can be degraded in the absence of oxygen. This suggests that it should be possible to design strategies for the remediation of MTBE within the anaerobic source zones of petroleum-contaminated aquifers, thus preventing further spread of MTBE contamination.

Joseph Robb and Ellen Moyer, ENSR International

Four petroleum retail marketing outlets with documented releases of petroleum hydrocarbons were evaluated to characterize and compare the natural attenuation of benzene and MTBE. Dissolved plumes of benzene and MTBE were characterized as stable, expanding or shrinking based on non-parametric Mann-Kendall trend evaluations. Concentration trends from individual monitoring wells were then interpreted within the hydrogeologic context of each site to describe site-wide trends in plume behavior. Datasets that did not exhibit upward or downward trends were further characterized as stable or non-stable trends with the Coefficient of Variation test. Attenuation rates were quantified using Sen’s non-parametric indicator of median slope. Geochemical parameters were reviewed to evaluate the potential contribution of biological degradation.

At Site A, Mann-Kendall trend analyses indicate decreasing trends in benzene and MTBE concentrations in 3 of 4 source area monitoring wells, with roughly equivalent attenuation rates for the two compounds. In addition, the downgradient extent of benzene and MTBE appear to be roughly equal. The comparable behavior of the benzene and MTBE plumes at Site A suggests MTBE and benzene are attenuating at similar rates. Site B showed evidence of a continuing source and steady state dissolved plumes of benzene and MTBE. At Sites C and D, the MTBE plume extends beyond the monitoring well network, but the MTBE plumes are shrinking and concentrations at the property boundary are low. The benzene plumes do not appear to extend past the property boundary at Sites C and D. Benzene appears to be attenuating more rapidly than MTBE at Sites C and D. Geochemical results at all sites are consistent with the consumption of oxygen for biological degradation of gasoline constituents. Natural attenuation could clearly play a role in site management at Sites A, C and D, while additional source control measures may be beneficial at Site B.

Michael Day and Terry Gulliver, Applied Hydrology Associates, Inc.

Tertiary butyl alcohol (TBA) is a natural degradation product of methyl tert-butyl ether (MTBE) and may be the most common indicator of degradation of MTBE in gasoline. TBA is typically regarded as slower to degrade than parent MTBE and thus may be the rate-limiting step in total mineralization of MTBE. Evidence for natural attenuation of TBA in groundwater is presented from a chemical plant in Pasadena, Texas. Several areas of the plant have shallow groundwater that has been affected by historic leaks and spills of TBA. A decade of regular groundwater monitoring of one groundwater plume, consisting primarily of TBA, shows generally declining concentrations and a plume area that is shrinking. Natural attenuation mechanisms are limiting the advective transport of TBA. Attenuation in this case is principally biodegradation, as the other physical components of natural attenuation (dilution, dispersion, diffusion, adsorption, chemical reactions, and volatilization) are relatively insignificant. This case history demonstrates that natural attenuation of TBA is important, and can be used as a groundwater management tool.

Dick Woodward, Sierra Environmental Services, Inc.

The Environmental Engineering Committee (EEC) of the EPA Science Advisory Board (SAB) released its review of Monitored Natural Attenuation: USEPA Research Program – An EPA Science Advisory Board Review (www.epa.gov/sab, fiscal year 2001 reports, EPA-SAB-EEC-01-004) in June 2001. Portions of the document addressed Monitored Natural Attenuation (MNA) of fuel oxygenates and MTBE. MNA’s position as a "knowledge-based" remedy focuses on the scientific and engineering knowledge used to understand and document naturally occurring processes rather than the imposed "active controls" of engineered remedies. The extent of the current knowledge base for ethers in the environment is similar to that of the knowledge base for chlorinated solvents ten years ago, although the former is advancing more rapidly. Concurrent with the release of the EEC review, reports of expanded, intrinsic biological capabilities surfaced in the refereed literature. These reports address some of the data gaps identified by the EEC and support the committee’s recommendations, namely: 1) to define the degradability of MTBE under broader field conditions, 2) to predict hydrocarbon mass fluxes leaving source zones, 3) to compile data on indirect evidence of fuel oxygenate natural attenuation and 4) to evaluate the risks of other, not-currently-listed fuel constituents. In addition to aerobic conditions, intrinsic biodegradation of MTBE has recently been documented under denitrifying conditions, iron reducing conditions, sulfate reducing conditions and methanogenic conditions. Comparative mass-flux considerations of fuel hydrocarbons and chlorinated solvents were discussed in detail in a recent ES&T paper. While evidence supporting the application of MNA to ether oxygenates is not as mature as that for BTEX, recent reports do support the concept of widespread natural attenuation capabilities, based primarily on intrinsic biodegradation, covering a range of electron acceptors and hydrocarbon substrates. The observation of numerous, phylogenetically diverse microorganisms capable of biodegrading MTBE worldwide suggests the capability is geographically widespread and that plume stabilization may be more related to population density and site conditions than to innate biological capabilities.