Methyl tert-butyl ether (MTBE) is a volatile organic compound (VOC) derived from natural gas that is added to gasoline either seasonally or year round in many parts of the

United States to increase the octane level and to reduce carbon monoxide and ozone levels in the air. In 1993, production of MTBE ranked second among all organic chemicals

manufactured in the United States. Currently, the U.S.

Environmental Protection Agency (EPA) tentatively classifies MTBE as a possible human carcinogen. Health complaints related to MTBE in the air were first reported in

Fairbanks, Alaska in November 1992 when about 200 residents reported problems such as headaches, dizziness, eye

irritation, burning of the nose and throat, disorientation, and

nausea. Similar health complaints have been registered in

Anchorage, Alaska; Missoula, Montana; Milwaukee,

Wisconsin; and New Jersey.

As part of the U.S. Geological Survey’s National WaterQuality Assessment (NAWQA) Program, concentrations of

60 VOCs were measured in samples from 211 shallow wells

in 8 urban areas and 524 shallow wells in 20 agricultural

areas. Chloroform and MTBE were the two most frequently

detected VOCs. MTBE was detected in 27 percent of the

urban wells and 1.3 percent of the agricultural wells. Concentrations ranged from less than the detection level of

0.2 μg/L (micrograms per liter) to as high as 23,000 μg/L.

When detected, the median concentration of MTBE was

0.6 μg/L. MTBE was most frequently detected in shallow

ground water in Denver, Colorado and urban areas in New

England. In Denver, 79 percent of the samples from shallow

urban wells had detectable concentrations of MTBE and in

New England, 37 percent of the samples from urban wells

had detectable concentrations. Only 3 percent of the wells

sampled in urban areas had concentrations of MTBE that

exceeded 20 μg/L, which is the estimated lower limit of the

EPA draft drinking water health advisory level. Contaminant

concentrations below the health advisory are not expected to

cause any adverse effects over a lifetime of exposure. MTBE

is on the EPA’s Drinking Water Priority List, which means it

is a possible candidate for future regulation.

What is MTBE and why is it used?

The Clean Air Act Amendments of 1990 mandate that

compounds that add oxygen (oxygenates) be added either

seasonally or year round to gasoline in specific parts of the

country where concentrations of ozone in the summer or carbon monoxide in the winter exceed established air-quality

standards. Oxygenates are added to increase the octane of

gasoline and to improve air quality in urban areas. Oxygenates are added to more than 30 percent of the gasoline in the

United States, and by the end of this decade, the Oxygenated

Fuels Association has estimated that oxygenates will be

added to 70 percent of the gasoline. MTBE is a commonly

used oxygenate because of its low cost, ease of production,

and favorable transfer and blending characteristics. It is

Occurrence of the Gasoline Additive

MTBE in Shallow Ground Water in

Urban and Agricultural Areas

made from methanol, which is derived primarily from

natural gas. Gasoline can contain up to 15 percent MTBE by

volume. In 1993, 24 billion pounds of MTBE worth about

$3 billion was produced in the United States. Domestic production of MTBE and its use in the United States decreases

the need for foreign oil.

Why is MTBE of interest?

About 109 million Americans live in counties where

MTBE is believed to be used (fig.1). Health complaints

related to MTBE in the air were first reported in Fairbanks,

Alaska in November 1992 when about 200 residents

reported headaches, dizziness, irritated eyes, burning of the

nose and throat, coughing, disorientation, and nausea after

MTBE had been added to gasoline. Health complaints also

have been registered in Anchorage, Alaska; Missoula,

Montana; Milwaukee, Wisconsin; and New Jersey. Studies

done by the Centers for Disease Control and Prevention in

Fairbanks, Alaska; Albany, New York; and Stamford,

Connecticut have shown that the concentration of MTBE in

the blood is related to the concentration of MTBE in the air.

People with the greatest exposure, such as gasoline service

station attendants and automobile mechanics, had the largest

concentrations of MTBE in their blood (ranging from less

than 0.05 to 37 μg/L); however, even commuters had

measurable concentrations of MTBE in their blood (ranging

from less than 0.05 μg/L to 2.6 μg/L). Furthermore, the

study in Fairbanks showed that among commuters there was

a significant increase in the concentration of MTBE in their

blood as a result of exposure to MTBE while driving.

Detectable concentrations of MTBE were found in the blood

of all those tested 2 months after the use of MTBE was suspended in Alaska.

What are the sources of MTBE?

All sources of MTBE released to the environment are not

well documented. The release of MTBE in 1992 from industry in the United States accounted for only 0.03 percent of

the MTBE that was produced. According to EPA’s Toxic

Release Inventory for 1992, about 94 percent of the MTBE

released from industry was released to the air, 3.5 percent

was discharged to surface water, and 2.5 percent was

injected into wells. Releases of MTBE in addition to those

from industry have not been quantified. For example, the

amount of MTBE released during refueling at service stations and from mobile sources such as vehicles is unknown,

but may be an important source of MTBE in the environment. Leaking underground storage tanks and spills at the

land surface are also sources of MTBE in the environment.

What are the chemical properties of MTBE

and its fate in the environment?

MTBE is an ether. It is a volatile, flammable, colorless

liquid at room temperature, and it smells like turpentine.

United States Geological Survey

National Water Quality Assessment Program
MTBE mixes with gasoline and is soluble in water, alcohol,

and other ethers. Because of its chemical characteristics,

MTBE would be expected to be found primarily in the atmosphere and in water. Results of a computer model used by

Environment Canada for southern Ontario showed that

56 percent of MTBE in the environment should be found in

the air, 43 percent in surface water and only about 0.5 percent in soil or streambed sediment. This model predicts

where MTBE may be found in the environment but needs to

be verified by environmental sampling. Although MTBE

will volatilize from soils, it is also highly mobile in soil and

can move into ground water. Once in ground water, MTBE

resists decay when compared to other gasoline components

like benzene. In surface water, MTBE is not expected to bioaccumulate in aquatic organisms.

It is hypothesized that MTBE moves with water in the

hydrologic cycle (fig. 2), but more data are needed to determine the extent of the movement. MTBE is released to the

air from sources such as industry and vehicles. Once in the

air, MTBE can mix with precipitation that may eventually

carry MTBE to the ground water or to streams. The MTBE

detected in snow samples collected in Denver, Colorado by

U.S. Geological Survey scientists supports this hypothesis.

Alternatively, gasoline spills may directly contribute to

MTBE contamination of ground water and surface water.

Where, how frequently, and at what concentrations is MTBE found in shallow

ground water?

The concentrations of MTBE and 59 other VOCs were

measured in samples of shallow ground water from 211

urban wells and 524 agricultural wells in 1993-94. These

monitoring wells are located in 8 urban and 20 agricultural

areas. These urban areas were located where MTBE was

released to the environment by industry or is potentially

used in gasoline. Some of the wells were constructed for the

NAWQA studies, whereas others were existing wells. Wells

for these studies were randomly located within specific landuse areas to allow comparison of shallow ground-water

quality with land use. Urban wells were located in industrial,

commercial, residential, and recreational areas, while agricultural wells were located in various crop areas.

Water-quality data from urban and agricultural areas show

that MTBE occurs predominantly in shallow ground water

underlying urban areas. MTBE was detected in 27 percent

of urban wells, and in 1.3 percent of agricultural wells distributed across the United States, with concentrations

ranging from less than the detection level of 0.2 μg/L to

23,000 μg/L. The concentrations of MTBE in ground water

from eight urban areas are shown in figure 3. When

detected, the median concentration of MTBE was 0.6 μg/L.

MTBE was detected in shallow ground water in all eight

urban land-use studies but was detected in ground water

from only 3 of 20 agricultural areas studied. For the urban

areas, MTBE was most frequently detected in Denver, Colorado, and in urban areas in New England (fig. 4). In Denver,

79 percent of the shallow urban wells (23 of 29 wells) had

detectable concentrations of MTBE, and in New England

(specifically urban areas within Connecticut, Massachusetts,

and Vermont), 37 percent of the wells (13 of 35 wells) had

detectable concentrations of MTBE. Other urban areas

where MTBE was detected included Reno, Nevada; Albany,

New York; Dallas/Fort Worth, Texas; Las Vegas, Nevada;

Atlanta, Georgia; and Albuquerque, New Mexico. Within

agricultural land-use areas, MTBE was detected in southern

Colorado, New England, and eastern Pennsylvania.

MTBE was the second most commonly detected VOC in

water from urban wells. Of the 211 urban wells tested,

28 percent had chloroform; 27 percent had MTBE; 18 percent had tetrachloroethene; 10 percent had trichloroethene;

7 percent had cis-1,2 dichloroethene; 5 percent had 1,1-

dichloroethane; and 5 percent had benzene. There are many

potential sources for these other chemicals; however, 1,1-

dichloroethane and benzene are used in gasoline, and chloroform has been identified in automobile exhaust.

Figure 1. Location of urban and agricultural areas studied, and locations where MTBE may be released and used.

NAWQA Urban Land-Use Studies

NAWQA Agricultural Land-Use Studies

MTBE Release Locations

Counties with potential use of MTBE

EXPLANATION

MTBE release locations from U.S. Environmental Protection Agency Toxic Release Inventory for 1992.

MTBE use areas from reformulated/oxygenated fuel program areas, U.S. Environmental Protection Agency.
Do the concentrations of MTBE in ground

water pose a threat to human health?

The EPA draft drinking water lifetime health advisory for

MTBE is estimated to fall within the range of 20-200 μg/L.

The health advisory is the maximum concentration in

drinking water that is not expected to cause any adverse

effects over a lifetime of exposure, with a margin of safety.

EPA expects to issue the final health advisory in the fall of

1995. EPA tentatively classifies MTBE as a possible human

carcinogen. MTBE is also on the EPA’s Drinking Water Priority List which means it is a possible candidate for future

regulation. There are no current Federal regulations that

require municipalities to test for MTBE in drinking water.

The water sampled by U.S. Geological Survey scientists

was located near the top of the water table and is the ground

water most likely to show contamination from sources at the

land surface. In seven of the eight urban areas studied, the

sampled ground water is the uppermost part of an aquifer

used for drinking water or is possibly connected to an underlying aquifer, which is used as a municipal water supply.

None of the urban wells sampled were being used as a

source of drinking water. In general, public water supplies

draw water from deeper parts of the ground water system

Figure 2. The movement of MTBE in the environment.

Precipitation

Emissions from

industry and vehicles

Overland runoff

Ground water

Clouds forming

Spills Precipitation

Leaking storage

tanks

River

Discharge to rivers

Figure 3. The concentrations of MTBE in each of the eight urban study areas.

0.1 1 10 100 1,000 10,000 100,000

Denver,

Colorado

New England

Reno,

Nevada

Albany,

New York

Dallas/Ft. Worth,

Texas

Las Vegas,

Nevada

Atlanta,

Georgia

Albuquerque,

New Mexico

DETECTED MTBE CONCENTRATION, IN MICROGRAMS PER LITER

Detection Level

Estimated range for

U.S. Environmental Protection Agency Health Advisory

(6)

(22)

(22)

(18)

(14)

(29)

(21)

(23)

(6) Number of samples with

 concentrations less than

 the detection level
and there are few data showing concentrations of MTBE at

these deeper depths. Of the urban monitoring wells tested,

about 24 percent had concentrations of MTBE ranging from

0.2 to 20.0 μg/L, and 3 percent had concentrations exceeding 20.0 μg/L.

What are the implications of this study?

NAWQA data show that MTBE is found predominantly

in shallow ground water in urban areas; however, many

questions need to be answered. For example:

(1) Can MTBE in shallow ground water be traced to nonpoint mobile sources, such as vehicle emissions? Alternatively, how much MTBE in shallow ground water is due to

point-source spills or leaking underground storage tanks?

(2) Do other fuel oxygenates occur in shallow ground

water in urban areas?

(3) What is the fate of MTBE and other possible oxygenates in shallow ground water; will these oxygenates degrade

over time due to natural processes, or will they accumulate

in ground water?

(4) What are the concentrations of MTBE and other oxygenates in the air, in precipitation, and in surface water in

urban areas? Is MTBE transported to ground water by infiltration of precipitation?

The U. S. Geological Survey will be working on these

questions in cooperation with city and state organizations,

and other Federal agencies.

Suggestions for further reading

Leahy, P.P., and Thompson, T.H., 1994, U. S. Geological

Survey National Water-Quality Assessment Program: U. S.

Geological Survey Open-File Report 94-70, 4 p.

Mormille, M.R., Liu, Shi, and Suflita, J.M., 1994,

Anaerobic biodegradation of gasoline oxygenates--

extrapolation of information to multiple sites and redox

conditions: Environmental Science and Technology, v. 28,

no. 9, p. 1727-1732.

Reichhardt, Tony, 1995, A new formula for fighting urban

ozone: Environmental Science and Technology, v. 29, no. 1,

p. 36A-41A.

Government of Canada, 1992, Canadian Environmental

Protection Act. Priority substances list, assessment report

no. 5, methyl tertiary-butyl ether: Ottawa, Canada, 19 p.

Moolenaar, R.L., Hefflin, B.J., Ashley, D.L., Middaugh,

J.P., and Etzel, R.A, 1994, Methyl tertiary butyl ether in

human blood after exposure to oxygenated fuel in

Fairbanks, Alaska: Archives of Environmental Health,

v. 49, no. 5, p. 402-409.

U.S. Environmental Protection Agency, 1994, Health Risk

Perspectives on Fuel Oxygenates. Office of Research and

Development; EPA report no. EPA/600/R-94/217,

Washington, D.C.

U.S. Environmental Protection Agency, 1993, Assessment

of Potential Health Risks of Gasoline Oxygenated with

Methyl Tertiary Butyl Ether (MTBE), Office of Research

and Development; EPA report no. EPA/600/R-93/206,

Washington, D.C.

—Paul J. Squillace, Daryll A. Pope, and Curtis V. Price

Information on technical reports and hydrologic data

related to NAWQA can be obtained from:

NAWQA VOC National Synthesis

U.S. Geological Survey, WRD

1608 Mt. View Rd.

Rapid City, SD 57702

Additional information on NAWQA and other U.S.

Geological Survey programs can be found by accessing

the NAWQA "home page" on the World Wide Web at

"http://wwwrvares.er.usgs.gov/nawqa/nawqa_home.

html."

Additional information on health effects of MTBE and

drinking water regulations can be obtained by calling

EPA’s Safe Drinking Water Hotline 1-800-426-4791.

Figure 4. The frequency of detection of MTBE for each urban study area.

March, 1995 U.S. Geological Survey, Fact Sheet FS-114-95

0 10 20 30 40 50 60 70 80 90 100

PERCENTAGE OF WELLS WITH MTBE

Denver,

Colorado

New England

Reno,

Nevada

Dallas/Ft. Worth,

Texas

Albany,

New York

Las Vegas,

Nevada

Atlanta,

Georgia

Albuquerque,

New Mexico

(23 of 29 wells)

(13 of 35 wells)

(6 of 28 wells)

(4 of 22 wells)

(3 of 17 wells)

(4 of 33 wells)

(2 of 23 wells)

(1 of 24 wells)