by: Douglas R. McCaulou and John C. Slater

Methyl tertiary-Butyl Ether (MTBE) may soon become a primary contaminant of concern for two reasons: 1) it is ubiquitous in the industrialized U.S., and 2) it is gaining regulatory attention because of its potential health hazards. MTBE has been used as a fuel additive since the late 1970’s and in 1993 MTBE was the second most produced organic compound (second only to gasoline) in the U.S.¹ Recently, MTBE has gained the public’s attention because of impacts on municipal groundwater supply wells and because of its possible health hazard.

MTBE is manufactured by combining methanol and isobutylene. It was introduced in the late 1970’s as an octane-enhancing lead replacement in gasoline (at concentrations up to 8%) and since, has been used at higher concentrations (11 to 15%) as a oxygenating agent in fuels. Because it is inexpensive, easy to manufacture, and boosts fuel octane, MTBE has become the fuel oxygenate of choice, but is likely to be phased out as such. The U.S. Environmental Protection Agency (EPA) tentatively classified MTBE as a possible human carcinogen in 1997². More recently, California has announced that MTBE exposure poses a potential cancer hazard. California and Maine have begun programs to eliminate MTBE from use in fuels in those states.

Regardless of future regulatory control of the use of MTBE, thousands of underground storage tank (UST) releases have impacted groundwater. Further, leaking USTs are not the only source of MTBE in water. Surface water impacts have been measured where atmospheric releases have resulted in dissolution of MTBE into the water phase3. The physical properties of MTBE make it especially problematic as a groundwater contaminant.

MTBE is highly soluble in water, migrates quickly, does not readily volatilize, does not adsorb well onto soil matter, and is generally recalcitrant to remediation.

• MTBE is very soluble in groundwater. Gasoline containing 10% MTBE could result in groundwater concentrations of MTBE of up to 5,000,000 micrograms per liter (µg/L)³. In contrast, the total gasoline hydrocarbon solubility in water is typically 120,000 µg/L.
• MTBE has a low Henry’s Law coefficient, that is, MTBE dissolved in water has low volatility.
• MTBE does not sorb well in aquifer systems. This allows it to move faster and further from the source than hydrocarbons (benzene, toluene, ethylbenzene, and xylenes [BTEX]).

These properties of MTBE are best observed in an MTBE/hydrocarbon groundwater plume. In the early stages of plume migration, MTBE and BTEX are co-mingled (right). However, they can quickly separate into discrete plumes because of their different chemical properties. MTBE moves readily with groundwater, whereas the movement of BTEX compounds is slowed by sorption. The relative retardation ratios in groundwater of MTBE, benzene, toluene, ethyl-benzene, and xylenes are 1:1:3:10:25:25, respectively. These different retardation ratios can result in considerable separation of these compounds, with the plume’s leading edge characterized by relatively high concentrations of MTBE, compared to the concentrations of other BTEX constituents. The figures above and below illustrate the transport separation between MTBE and BTEX from a gasoline release to groundwater.

These properties of MTBE also make it expensive to treat. For example MTBE’s high solubility and low volatility dramatically hinders its removal by air stripping. Its reduced adsorption to granular carbon increases the carbon consumption rate in both groundwater and air stripper off-gas treatment systems. In Part 2 of this series, we will discuss treatment alternatives for groundwater containing MTBE.

Field observations suggest that MTBE is recalcitrant to both aerobic and anaerobic degradation in a groundwater environment and that natural attenuation is mainly limited to dilution and dispersion⁴. Unlike BTEX contamination, where natural biodegradation will slow the advance of a plume, MTBE has the potential to move farther away from its source with groundwater and may persist much longer. Although, recent laboratory studies indicate that MTBE may biodegrade in engineered laboratory systems, quantifying the potential in-situ biodegradation of MTBE is currently a hot research topic.

No enforceable federal drinking water standard exists for MTBE. In the long-term, EPA added MTBE to the Final Drinking Water Contaminant Candidate List that is required under the Safe Drinking Water Act⁵. Further, a Blue-Ribbon Panel formed by EPA has recommended tightening regulations for UST monitoring and leak detection, expanding regulation of fuel storage systems, and accelerating drinking water source protection efforts⁶.

Several states have set guidelines and enforceable standards for MTBE in drinking water supplies (table below). Most notably, California has established an enforceable secondary maximum contaminant level (MCL) of 5 µg/L and has set a tentative primary MCL of 13 µg/L. The current groundwater clean up goals range from 5 µg/L in some states to no-required-action in others. The future of thousands of gasoline-contaminated groundwater sites, including sites considered closed by current standards, may be affected by further tightening of regulatory standards.

Currently, MTBE remediation is not a goal of groundwater remedial efforts in most states. However, this may change as other states adopt California’s stance. HGC will continue to monitor and report changes in regulations, technologies, and other issues that affect on-going and future groundwater remedial actions. Look for a discussion of remedial alternatives for MTBE in Part 2 of this series.

¹Reisch, M. S. 1994. Top 50 chemicals production rose modestly in the U.S. Chemical and Engineering News, 72: 12-15.

²EPA. 1997. Drinking Water Advisory: Consumer Acceptability Advice and Heath Effects Analysis on Methyl Tertiary-Butyl Ether (MTBE). EPA-8220-F-97-009. ODW 4304.

³Squillace, 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.

⁴Landmeyer, J.E., F.H. Chapelle, P.M. Bradley, J.F. Pankow, C.D. Church, and P.G. Trattnyek. 1998. Fate of MTBE relative to benzene in a gasoline contaminated aquifer. Ground Water Monitoring & Remediation, Fall 1998, pp 93-102.

⁵EPA. 1997. 62 FR 52193 40 CFR Pts 141, 142, Announcement of Draft Drinking Water Contaminant Candidate List. Federal Register 62(128): 52193-52219. Monday, October 6, 1997.

⁶EPA. 1999. The Blue-Ribbon Panel on Oxygenates in Gasoline: Panel Findings - Executive Summary and Recommendations. July 27, 1999.