Methyl tert-butyl ether (MTBE) is an oxygenated compound which has been increasingly used in gasoline in U.S. urban areas to improve combustion and reduce the levels of atmospheric carbon monoxide, hydrocarbons, and ozone. The 1990 Clean Air Act requires the use of oxygenated luels in areas which exceed the national standard for carbon monoxide. Gasoline containing MTBE as the only oxygenated component may contain 15% MTBE by volume.

MTBE has greater water solubility than aromatic compounds; water in equilibrium with gasoline containing 15 vol.% MTBE would contain 9,500 ppm¹. As a result, it moves more readily into ground water than benzene, toluene, ethylbenzene and xylenes (BTEX), and can create a "halo effect" around a source. MTBE has an average odor detection threshold in water in the range of 45 to 95 ppb and has an average taste threshold of 134 ppb. Because of its low taste and odor threshold, the Oxygenated Fuel Association thinks it likely that more leaking tanks will be discovered.

Since MTBE has only recently been found extensively in ground water, the regulation of MTBE contamination is still evolving. The USEPA is expected to establish a lifetime health advisory for MTBE at either 20 ppb or 200 ppb, depending on whether it is listed as a possible human carcinogen. Because it moves relatively rapidly in ground water, MTBE may confound attempts to remediate leaking underground tanks using "intrinsic bioremediation." MTBE is also not included in the examples of Risk-Based Screening Levels used in the ASTM Risk-Based Corrective Action Standard for petroleum releases.

A recent U.S. Geological Survey report found that (MTBE) is the second most frequently detected chemical in shallow urban monitoring wells. They found that MTBE was detected in 27% of shallow monitoring wells and springs in urban areas. USGS found no MTBE in drinking water wells. The USGS used a sensitive GC-MS technique with a reporting level of 0.2 ppb; most of the detected MTBE was below 20 ppb.

Interestingly, the USGS found that the occurrence and concentration of MTBE did not correlate with BTEX in ground water. They offered two explanations for this lack of con'elation. The first is that MTBE is derived from point-source leaks or other discharges. Because it is relatively water soluble, it dissolves readily in ground water and moves faster than BTEX or any other fuel component. The second explanation offered by USGS is a non-point source of MTBE in the atmosphere, resulting from leaks, fueling of automobiles, and spills. Once in the atmosphere, MTBE would dissolve in rainwater and contaminate surface water or shallow ground water.

The occurrence of MTBE has prompted regulatory agencies to add it to existing monitoring programs. For example, the San Francisco Regional Water Quality Control Board sent a letter in July to local agencies and laboratories on MTBE reporting. The Board recommended a reporting limit of 5 ppb for MTBE by USEPA Method 602 or 8020. Initially, some laboratories indicated that MTBE could be added to the target compounds in 602 or 8020 without any additional charge. However, recently some labs have found very high levels of MTBE relative to BTEX. This requires an additional dilution and analysis to quantitate MTBE. The SFRWQCB is requiring collection of MTBE data to assess biodegradation, fate and transport, and potential human exposure. The Board is not requiring remediation specifically for MTBE, nor is it used in site closure at this time.

In October, Jeff Sickenger, the Environmental Issues Coordinator of the Western States Petroleum Association (WSPA), wrote a letter to John Farr, the Chair of the SB1764 Advisory Committee, in which he expressed concern that MTBE reporting by USEPA Method 8020 (gas chromatography - photo ionization detection, GC-PID), may require confirmation. Many labs in California now do "TPH" analysis with a combination of gas chromatography with flame-ionization (GC-FID) and GC-PID. MTBE can be measured with the same technique, although there is some concern about the identification of MTBE. WSPA is concerned that MTBE may not be correctly identified by GC only, and that MTBE presence should be confirmed by gas chromatography - mass spectrometry (GC-MS), for example, with Method 8260, at least once per site. MTBE is not a target compound in Method 8260, but it can be measured by that method. At least some of the Regional Water Quality Control Boards are adopting the guidance on MTBE, and will be requiring that MTBE be added to TPH and BTEX analysis, and requiring the MTBE confirmation by GC-MS at least once per site.

In a future column, we plan to discuss the status of risk-based measurements for petroleum releases, and what role MTBE may play in risk-based corrective action.

The problem of accurately measuring MTBE is being resolved. The environmental behavior of MTBE needs better understanding before we understand the sources; the assessment and remediation of MTBE contaminated plumes are problems which will require the concerted efforts of all concerned.

Barton P. Simmons, Ph.D., is an environmental scientist with the Hazardous Materials Laboratory, California Department of Toxic Substances Control in Berkeley.

1. "MTBE in Ground Water, Fact Sheet for Local Health and Water Authorities," Oxygenated Fuels Association, March, 1995.
2. Squillance, Paul J., et al "A Preliminary Assessment of the Occurrence and Possible Sources of MTBE in Ground Water of the United States, 1993-94," U.S. Geological Survey Open-File Report 95-456, 1995. Copies of the report are available from: U.S. Geological Survey, Earth Science Information Center, Open-File Reports Section, Box 25286, MS 517, Denver, CO 80225-0425.