Environmental Behavior and Fate of MTBE, Part 1. Some MTBE background

Water can dissolve large amounts of MTBE from gasoline that has been oxygenated with methyl tertiary-butyl ether (MTBE). At 25 ºC the water solubility of MTBE is about 5000 mg/L for a gasoline that is 10% MTBE by weight. In contrast, the total hydrocarbon solubility of gasoline in water is typically about 120 120/mg/L. MTBE sorbs only weakly to soil and aquifer materials. Therefore, sorption will not significantly retard MTBE's transport by groundwater. In addition, the compound generally resists degradation in groundwater.

Fuel oxygenates are voluntarily added to gasoline to enhance the octane of gasoline in many areas of the U.S. They have been used since 1988 to improve air quality in some metropolitan areas. The 1990 Clean Air Act Amendments since November 1, 1992 have required oxygenated gasoline in areas that exceed the national ambient air-quality standard for carbon monoxide (carbon monoxide non-attainment areas) during the winter when the concentrations of carbon monoxide are highest. Furthermore, since January 1995, the 1990 CAA amendments have required nine metropolitan areas with the most severe ozone pollution to use, year-round, reformulated gasoline that contains fuel oxygenates. Numerous additional metropolitan areas have chosen to participate in the oxygenated fuels and reformulated gasoline programs.

According to a U.S. Environmental Protection Agency (EPA) statement in 1994, oxygenates were added to more than 30% of the gasoline used in the U.S. and the percentage was expected to increase in the future. Indeed, it has been projected that by the year 2000 fuel oxygenates will be added to 70% of the gasoline used in the U.S.

Its low cost, ease of production, and favorable transfer and blending characteristics make MTBE the most commonly used fuel additive. It can be produced at the refinery, it blends easily without separating from gasoline, and the MTBE-gasoline blend can be transferred through existing pipelines. In the U.S., almost all MTBE is used in gasoline. The second most-used fuel oxygenate is ethanol. Other oxygenates in limited commercial use include methanol, ethyl tertiary-butyl ether, tertiary-amyl methyl ether, and diisopropyl ether.

The EPA has tentatively classified MTBE as a possible human carcinogen, but no drinking-water regulation has been established for the compound. The EPA, however, has issued a drinking water advisory of 20 to 40 µg/L (micrograms per liter). This advisory is based upon taste and odor thresholds. However, this advisory concentration provides a large margin of safety for non-cancer effects, and is in the range of margins typically provided for potential carcinogenic effects.

MTBE has been detected in groundwater and storm water. During its 1993-94 National Water Quality Assessment program, the U.S. Geological Survey analyzed 60 volatile organic compounds (VOCs) in samples of shallow ambient groundwater from eight urban areas. It found MTBE to be the second most frequently detected compound (after trichloromethane, [chloroform]).

The groundwater samples were collected from 5 drinking water wells, 12 springs, and 193 monitoring wells. At a reporting level of 0.2 µg/L, MTBE was detected in water from 27% of the 210 wells and springs sampled, but no MTBE was detected in water from the drinking-water wells.

MTBE was the seventh most frequently detected VOC, occurring in 6.9% of the storm-water samples. Measurable concentrations of MTBE also were found in some of 592 storm-water samples collected by the USGS survey in 16 cities and metropolitan areas required to obtain National Pollutant Discharge Elimination System (NPDES) permits.

Only about 20% of the MTBE samples were analyzed for levels as low as 0.2 µg/L. Generally, MTBE was reported for levels exceeding 1.0 µg/L. This higher level likely resulted in fewer incidents of detection compared with some of the other VOCs, all samples of which had a reporting level of 0.2 µg/L. Where MTBE was detected, concentrations ranged from 0.2 to 8.7 µg/L.

Eighty-three percent of all storm-water samples that had detectable concentrations of MTBE were collected between October 1 and March 31. This is the approximate period when oxygenated gasoline is used in carbon monoxide non-attainment areas.

Part 2 of this presentation will cover MTBE's partitioning to air, water, and subsurface solid.

About the Authors: Paul J. Squillace is a hydrologist with the U.S. Geological Survey (USGS) in Rapid City, SD. John S Zogorski also is a member of the staff at the same USGS location. James F. Pankow works for the Oregon Graduate Insttiute for Science and Technology, and Nic E. Korte is employed at the Oak Ridge National Laboratory in Tennessee.

Additional information on this article, and about NAWQA and other USGS programs, can be found by accessing the NAWQA web site at: http://wwwrvares.er.usgs.gov/nawqa/nawqa_home.html.

Or contact author Paul Squillace directly at: U.S. Geological Survey, 1608 Mountain View Rd., Rapid City, SD 57702; Tel. 605-355-4560 (ext. 239); e-mail at pjsquill@usgs.gov

The above presentation is adapted from a feature article appearing on the companion VerticalNet Online publication, Water Online.