James A. M. Thomson, Applied Hydrology Associates, Inc., Denver, Colorado

Background

Early investigations into the biodegradability of MTBE under natural conditions were not very promising (e.g., Horan and Brown, 1995). In fact, until quite recently, MTBE was considered, primarily on the basis of laboratory microcosm tests, to be “…recalcitrant to both aerobic and anaerobic biodegradation conditions” (Odencrantz, 1998). According to the USGS, “MTBE is generally reported as recalcitrant …” (Squillace et al, 1998).

However, as results of more recent research have been published, data have started to accumulate that indicate that, under the right conditions, MTBE will biodegrade intrinsically. Last year’s AEHS special MTBE issue contained the prediction (Thomson, 2000) that prospects for remediation of MTBE-affected sites through complete source removal (SR) and passive or enhanced monitored natural attenuation (MNA) were good. This assertion was based on a number of supporting case histories and field evidence, indicating the following:

• Comparable MTBE and benzene plume behavior.

• Stabilization, reduction, or disappearance of MTBE plumes.

• Reduction in formerly stable MTBE plume concentrations following source removal.

• Incorrect model predictions when “fed” with the assumption of no MTBE biodegradation.

In the past year, new developments have supported the potential for intrinsic bioremediation of MTBE. This paper, based on an extensive literature review, summarizes some of the key advances made during 2000. Other authors have also presented summaries that may include data not presented here. For example, Deeb et al (2000) presented a review of biodegradability of MTBE and TBA, including data from lab microcosm experiments, ex-situ bioreactor studies, and controlled field trials. A brief summary is also presented in API (2000).

The potential for MNA is one that is of great interest at most regulatory levels; the official federal position is conservative, but strongly promotes additional research. Thus, the EPA’s Science Advisory Council’s (SAC, 2000) statements that: “… the most pressing need is in the area of MTBE biodegradation”, and “More research is sorely needed to predict the potential for MTBE biodegradation”, particularly under anaerobic conditions such as occur as most LUST sites. 

Aerobic biodegradation

The limited detection of MTBE in surface waters, despite its common detection in urban precipitation, suggests that aerobic breakdown is a significant process. Bradley et al (1999, 2001) collected microorganism-containing stream and lakebed sediment samples from 11 sites throughout the United States. Sample sites included waters exposed to MTBE or other chemicals, as well as unaffected springs. These samples were used to conduct microcosm studies using isotopically labeled MTBE. Every sediment sample examined demonstrated significant aerobic mineralization of MTBE within 50 days, as indicated by the increase in the percentage of labeled carbon in the biomass.

Kane et al (2000) performed microcosm experiments using soil and groundwater from MTBE-affected LUST sites. Results showed relatively rapid degradation of MTBE under aerobic conditions in some sediments and no degradation in similar sediments. Addition of a growth medium enhanced MTBE degradation rates. MTBE degradation was significantly inhibited and TBA persistence increased, by the presence of dissolved gasoline constituents, notably BTEX.

Kuhn (2000) reported field data from an approximately 4,000-gallon gasoline leak at the Ronan LUST Trust site in Ronan, Montana. A mixed BTEX/MTBE dissolved plume extended over 2,000 feet downgradient from the source; the LNAPL plume was addressed by soil and product recovery, air sparging, and SVE. Initial soil samples collected at the site contained an aerobic microbial consortium capable of degrading MTBE under lab conditions. However, the main body of the plume is anaerobic. The leading edge of the plume discharges to surface water under aerobic conditions, and MTBE is degraded by in-situ bacteria at this interface; however, the rate of removal does not appear to be adequate to remove all the dissolved MTBE.

The pioneering work by Doug MacKay (2000) at Vandenberg Air Force Base, California, has shown that establishment of aerobic subsurface conditions using a bubbleless oxygen curtain can result in complete degradation of MTBE within the order of a few days or less. Investigation of indigenous microbes at the site by Kate Scow suggests that they may include a match with the rapid MTBE degrader known as PM-1 detected at Port Hueneme and isolated at her UC Davis laboratory (Church et al, 2000).

Similarly, Salanitro, et al (2000) showed that aquifer “seeding” (i.e., bioaugmentation) with a proprietary microbial consortium known as MC-100 (marketed by Equilon as BioRemedy), followed by subsurface oxygenation, resulted in degradation of MTBE at 5‑80 mg/L concentrations.

As a result of these two major field studies, interest in both forced aerobic bioremediation and bioaugmentation has greatly increased. Typically, laboratory studies have shown lower rate constants, longer half-lives, and less promising results than field data. For example, Drogos and Diaz (2000) used model column aquifers to investigate MTBE degradation under controlled conditions. They found that MTBE degraded to TBA after a 35-day lag period, but only under aerobic conditions and in the absence of BTEX. Degradation declined as dissolved oxygen was depleted. The aerobic condition was further investigated to determine whether the degradation rate could be enhanced by the addition of a cometabolic substrate. However, it was found that conversion of MTBE to TBA stopped on the addition of the substrate (isopropanol, hexane, isopentane, isopentanol, malate, and ethanol), presumably because they are more favorable to the degrader than MTBE. The important implication is that competing substrates (including BTEX and ethanol) may inhibit MTBE biodegradation at some sites. From the combined results of this study, the average half-life for MTBE biodegradation was between 1.7 and 2.7 years, depending on the sediment type.

Several vendors have presented innovative equipment that can be used to force aerobic conditions to enhance in-situ bioremediation. For example, Matrix Environmental Technologies (2000) has patented an oxygen-enhanced bioremediation system that supplies pure oxygen gas to the subsurface, resulting in higher dissolved oxygen concentrations, promoting aerobic biodegradation. The system is only for use in relatively high permeability formations, not silt or clay. Regenesis Corporation (2000) reports that the use of its Oxygen Release Compound (ORCã) accelerates MTBE biodegradation, increasing rate constants from the range 0.0038-0.0231/d to 0.0112-0.1447/d (i.e. reducing half-lives from 30-182 days to 5-61 days).

The bioaugmentation field has not remained static. Envirogen, Inc. (2001) recently announced that it has been awarded a $0.5M grant by the National Science Foundation (NSF) to continue its work on in-situ remediation of MTBE through bioaugmentation. Envirogen has also developed reactor-based systems to degrade MTBE and the related compound tert-butyl alcohol (TBA).

Anaerobic biodegradation

While Mackay and Salanitro’s results have encouraged interest in aerobic conditions, ambient conditions at most LUST sites are more typically anaerobic than aerobic, and anaerobic degradation is generally easier to engineer in the field than aerobic. Therefore, biodegradation of MTBE under anaerobic conditions remains of great interest. While field conditions are more complex than in the laboratory, empirical field data do reflect the real world, and their results are received as particularly applicable to remediation projects. Kolhatkar et al (2000) addressed the question of biodegradability of MTBE in the subsurface by conducting comprehensive groundwater chemistry surveys at 74 gas station sites in 6 states and D.C. in 1999. These data were used to estimate first order biodegradation rate constants for MTBE, TBA, and benzene. Apparent biodegradation rates could only be established (i.e., were statistically significantly different from zero) for 4 out of 74 sites. The rate constants for MTBE varied from 0.41 to 9.9 per year; for TBA, 5.5 to 12.8 per year; and for benzene, 2.3 to 3.3 per year. MTBE and TBE biodegradation rates were similar to those for benzene.

There appeared to be a good correlation between strongly anaerobic plume geochemistry and natural MTBE biodegradation. Degradation of MTBE and TBA was limited to sites that were classified as methanogenic (dissolved methane >0.5 mg/L), and most were sulfate depleted relative to background concentrations. None of the sites with <0.5 mg/L dissolved methane showed MTBE biodegradation. This study is continuing, and additional results are anticipated.

Wilson et al (2000) performed a very detailed evaluation of intrinsic bioremediation of MTBE under methanogenic conditions at a former US Coast Guard fuel farm site in Elizabeth City, PA. At this site, hydrocarbon metabolism is primarily through anaerobic pathways (sulfate and iron oxidation, and methanogenesis). Field data indicated that MTBE was being naturally attenuated with rate constants between 2.2 and 5.0/yr (depending on the calculated groundwater seepage velocity), while attenuation due to dilution and dispersion was estimated at only 0.50/yr.

In this study, microcosms were constructed using aquifer material from the existing monitoring well location with the highest concentration of MTBE; material was sampled so as to preserve anaerobic conditions. Samples were amended either (1) with MTBE alone, or (2) with MTBE and various alkylbenzenes. Microcosms were incubated for approximately 6, 13, and 16 months. The average first order rate of removal of MTBE was 3.02/yr where supplemented with alkylbenzenes, 3.5/yr without alkylbenzenes, and 0.39/yr and 0.30/yr in the corresponding controls (all at 95% confidence).

It was concluded that the apparent intrinsic bioremediation was consistent with the biodegradation rates expected from microcosm studies, and that it would take approximately 60 years for the concentration to reach 30 ppb.

Conclusions

While the incoming evidence clearly supports the occurrence of intrinsic bioremediation of MTBE, it also indicates that important issues remain to be resolved, including:

• Degradation pathways of MTBE and its derivatives.

• “Lag” time in starting biodegradation.

• Potential inhibition due to competition from other substrates.

• Potential inhibition due to toxicity of other chemicals or matrices.

• To what extent intrinsic bioremediation can be accelerated by addition of nutrients, substrates, and electron acceptors (i.e. moving toward engineered remediation).

• Ability to create successful MNA conditions in typical field situations.

With the current focus on remedial options for MTBE, and the emphasis that is being placed on low-cost in-situ technologies, it is expected that research into these aspects of MTBE intrinsic bioremediation will continue to flourish and that new revelations will be forthcoming in the year ahead.

References

American Petroleum Institute. 2000. Strategies for characterizing subsurface releases of gasoline containing MTBE. Regulatory and Scientific Affairs Publication No. 4699. February 2000. Referenced pages A-10 to A-11.

Bradley, P. M., J. E. Landmeyer, and F. H. Chapelle. 1999. Aerobic mineralization of MTBE and tert-Butyl Alcohol by stream-bed sediment organisms. Environmental Science and Technology, vol. 33, no. 11, pp. 1877-1879.

Bradley, P. M., J. E. Landmeyer, and F. H. Chapelle. 2001. Widespread potential for microbial MTBE degradation in surface-water sediments. Environmental Science and Technology. Accepted November 28, 2000.

Church, C. D., P. G. Tratnyek, and K. M. Scow. 2000. Pathways for the degradation of MTBE and other fuel oxygenates by isolate PM1. American Chemical Society Symposium, San Francisco, March 26-30, 2000. pp. 261-263 in Preprints of Extended Abstracts, vol. 40, No. 1.

Deeb, R. A., A. J. Stocking, L. Alvarez-Cohen, and M. C. Kavanaugh. 2000. MTBE and TBA biodegradation: a current review. In Proceedings of the 2000 Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Remediation: Special Focus: Natural Attenuation and Gasoline Oxygenates. November 15-17, 2000, Anaheim, California. National Ground Water Association. pp. 50-51 (abstract).

Drogos, D. L., and A. F. Diaz. 2000. Exploring the environmental issues of mobile, recalcitrant compounds in gasoline. American Chemical Society Symposium, San Francisco, March 26-30, 2000. pp. 238-240 in Preprints of Extended Abstracts, vol. 40, No. 1.

Envirogen, Inc. 2001. Press Release: Envirogen Announces New MTBE Contract. January 19, 2001.

Horan, C. M. and E. J. Brown. Biodegradation and inhibitory effects of methyl-tertiary-butyl ether (MTBE) added to microbial consortia. In Proceedings of the 10th Annual Conference on Hazardous Waste Research. May 23-24, 1995, Kansas State University, Manhattan, Kansas. Great Plains-Rocky Mountain Hazardous Substance Research Center

Kane, S., H. Beller, T. Legler, C. Koester, and A. Happel. 2000. Evaluation of MTBE biodegradation in commercial LUFT sites: microcosm studies. In Proceedings of the 2000 Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Remediation: Special Focus: Natural Attenuation and Gasoline Oxygenates. November 15-17, 2000, Anaheim, California. National Ground Water Association. p. 116 (abstract).

Kolhatkar, R., J. Wilson, and L. E. Dunlap. 2000. Evaluating natural biodegradation of MTBE at multiple UST sites. In Proceedings of the 2000 Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Remediation: Special Focus: Natural Attenuation and Gasoline Oxygenates. November 15-17, 2000, Anaheim, California. National Ground Water Association. pp. 32-49.

Kuhn, J. 2000. Natural attenuation of MTBE at the Ronan LUST Trust site, Ronan, Montana. In Proceedings of the 2000 Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Remediation: Special Focus: Natural Attenuation and Gasoline Oxygenates. November 15-17, 2000, Anaheim, California. National Ground Water Association. p. 244 (abstract).

Mackay, D., C. Naas, R. Wilson, K. Scow, D. Ghandi, A. Smith, and M. Einarson. 2000. Field tests of enhanced intrinsic remediation of an MTBE plume at Vandenberg Air Force Base. Unpublished Progress Update. October 2, 2000. 6 pp.

Matrix Environmental Technologies. 2000. Oxygen-enhanced bioremediation. US Patent No. 5,874,001.

Odencrantz. J. E. 1998. Implications of MTBE for intrinsic remediation of underground fuel tank sites. Remediation, Summer 1998. pp. 7-16.

Regenesis Corporation. 2000. ORC Technical Bulletin #2231. 4 pp.

Salanitro, J.P., P. C. Johnson, G. E. Spinnler, P. M. Maner, H. L. Wisniewsky, and C. Bruce. 2000. Field-scale demonstration of enhanced MTBE bioremediation through aquifer bioaugmentation and oxygenation. Environmental Science and Technology, vol. 34, pp. 4152-4162.

Science Advisory Board. 2000. Subject: EPA’s Natural Attenuation Research Program, Draft Subcommittee Report for Environmental Engineering Committee meeting, December 5-7, 2000. November 30, 2000. 64 pp.

Squillace, P. J., J. F. Pankow, N. E. Korte, and J. S. Zogorski. 1998. Environmental behavior and fate of methyl tert-butyl ether (MTBE). USGS Fact Sheet FS-203-96 (Revised 2/98).

Thomson, J. 2000. Prospects for natural attenuation of MTBE. Soil Sediments and Groundwater. MTBE Special Issue. March 2000. pp. 41-42.

Wilson, J. T., J. S. Cho, B. H. Wilson, and J. A. Vardy. 2000. Natural attenuation of MTBE in the subsurface under methanogenic conditions. National Risk Management Research Laboratory, Ada, Oklahoma. EPA/600/R-00/006. January 2000. 49 pp.

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