Jim Barker, Department of Earth Sciences, University of Waterloo
Methyl-tertiary-Butyl Ether (MTBE) is added to gasoline at 2 to 15% to improve its combustion performance, especially to lower ozone and carbon monoxide levels in urban areas. It appears to be a popular additive - 6.2 billion kg were produced in the U. S. and production has increased about 25% annually since 1984. It is likely present in almost all U. S. gasoline and its use in Canada is increasing. One advantage of MTBE is that it can replace aromatic hydrocarbons such as benzene as a major contributor to octane enhancement. This has a potential environmental benefit in that benzene is a carcinogen and MTBE is not very toxic. In fact, following its 1992 assessment under the Canadian Environmental Protection Act, MTBE was not considered to be toxic as defined under section 11 of the Act.
Gasoline manufacture, distribution, and storage causes gasoline to impact water, in addition to air quality. Perhaps the major environmental impact of gasoline is upon soils and groundwater. Gasoline enters the environment via pipeline ruptures, leakage from underground storage tanks and piping, and from surface spills. When this gasoline contacts groundwater or infiltrating water, the water-soluble gasoline components are preferentially leached into this water and the mobile components can migrate rapidly in the groundwater towards receptors such as drinking water wells, streams, and wetlands. Monoaromatic hydrocarbons such as benzene, ethylbenzene, and the three xylene isomers are the most water-soluble, mobile and toxic hydrocarbon components (drinking water standards typically 5 to 100 micrograms per litre (ug/L). They often occur at 1 to 50 milligrams per litre (mg/L) levels in groundwater near leaking sources and so their fate has been emphasized in cases of gasoline-contaminated groundwater. MTBE is even more water-soluble and very mobile in groundwater and so it is typically found in 1 - 400 mg/L quantities in groundwater impacted by MTBE-containing gasoline. However, its low toxicity would appear to make it of little concern in groundwater.
Research by Earth Sciences faculty and graduate students at Waterloo has clarified the behaviour of these organic compounds in groundwater. Much of this research took advantage of the research aquifer established at Canada Forces Base Borden. Releases of small volumes of water and/or gasoline into a shallow part of this aquifer have permitted the researchers to establish the precise input of contaminants to groundwater. One of our experiments, from 1988 to 1990 included MTBE in the injected water (Tina Hubbard, MSc thesis). By monitoring the migration and fate of chemicals using an extensive network of over 6000 miniwells, the researchers established that the mobile monoaromatics are readily biodegraded to innocuous products by naturally-occurring aerobic bacteria. This biodegradation will likely limit the extent of groundwater contamination by benzene, etc. at many gasoline spills/leaks. On the other hand, MTBE appears to be mobile and non-degraded in groundwater. However, dispersive dilution will likely reduce MTBE concentrations to less-than-toxic levels (perhaps 200 - 500 mg/L) not too far from most sources.
In the 1980's few regulators found the scientific basis sufficient to permit the natural dispersive dilution and biodegradation processes to be used to clean up groundwater contaminated by gasoline. During this time, other remediation technologies, such as pumping out and treating the contaminated groundwater or adding oxygen and nutrients to promote in situ biodegradation of monoaromatics, showed limited success, with various technical and costs challenges. In the 1990's, the natural remediation is being tried. This strategy is termed "do nothing but monitor" or "intrinsic remediation". Where the natural remediation appears to be sufficient for the desired risk management or groundwater resource protection, confirmatory monitoring is really all that is required. This technology has obvious cost appeal.
Along comes MTBE. No problem, you say, since MTBE is not really toxic. However, MTBE has a strong odour and taste. It is not the worst smell you have encountered, but you certainly wouldn't want to drink water containing more than 10 - 100 ug/L of MTBE. And that is the problem: natural or intrinsic processes are not likely to reduce MTBE concentrations in gasoline-impacted groundwater to such low levels. If large volumes of groundwater, far-removed from the source of contamination are not usable, then intrinsic remediation is not sufficient and more active (and costly) remediation may be required.
So, just as we begin to make use of Nature's ability to clean up gasoline contaminants in groundwater, we realize we have added a chemical to gasoline that may preclude Nature's cost-effective solution for groundwater contamination. So, while MTBE has a major environmental appeal with respect to air quality, it is an environmental liability for groundwater. I guess that is a typical environmental situation - there are both good and bad sides.
In the mean time, the U. S. Geological Survey has conducted a survey for MTBE occurrence in groundwater in the U. S. (Open File Report 95-456). Of 210 urban wells and springs sampled, 27% contained MTBE, none at levels causing health concern, and few at levels causing nuisance. These results certainly raised concerns about our use of MTBE in gasoline. At Waterloo, we are going back to the groundwater plume of MTBE we created at Borden in 1988 to see if we can find evidence of MTBE biodegradation. Our initial 16 months of monitoring found that MTBE was apparently persistent, but we need to establish the longer-term behaviour of MTBE, especially as its apparent recalcitrance may drive expensive, active remediation of groundwater. It is likely that even slow biodegradation of MTBE in groundwater, with a MTBE half-life of even 5 years, will permit intrinsic remediation to be successful in many cases. We hope to have our answer in late-1996.
References
Hubbard, C.E., 1992. Transport and fate of dissolved methanol, methyl-tertiary-butyl-ether and monoaromatic hydrocarbons in a shallow sand aquifer. Unpublished M.Sc. thesis, University of Waterloo.
Squillace, P.J., Zogorski, J.S., Wilber, W.G. and Price, C.V., 1995. A preliminary assessment of the occurence and possible sources of MTBE in ground water of the United States, 1993-94. U.S. Geological Survey Open-File Report 95-456.