Gasoline Additive Found In Urban Stormwater Runoff and Ground Water, But Levels Are Low

November 21, 1996

The gasoline additive MTBE (methyl tert-butyl ether) was detected in some urban stormwater samples collected in 16 cities and metropolitan areas by the U.S. Geological Survey, but all detections of MTBE were less than the lower limit of the U.S. Environmental Protection Agency's draft lifetime health advisory for drinking water.

In another study as part of the USGS National Water Quality Assessment Program, MTBE was detected in 24 percent of ground-water samples collected from wells in New England, but again the levels were at low concentrations.

MTBE and its potential effects on water resources were the focus of a session Thursday (Nov. 21, 1996) at the 17th annual meeting of the Society of Environmental Toxicology and Chemistry in Washington, D.C. USGS scientists reported on their various studies at the session.

MTBE, derived from natural gas, is added to gasoline in many parts of the United States to increase the octane level or to reduce carbon monoxide and ozone levels in the air. The U.S. Environmental Protection Agency tentatively classifies MTBE as a possible human carcinogen, which has prompted continued study of its effects on the environment.

USGS scientists said MTBE was detected in one or more stormwater samples in eight cities -- Atlanta, Baton Rouge, Birmingham, Colorado Springs, Denver, Dallas/Fort Worth, San Antonio and Phoenix. The detection rate in urban stormwater was highest (34 out of 41 samples) in samples collected during the months of October through March each year of the study (1991-95). This October-March detection window corresponds with the expected seasonal use of oxygenated gasoline in areas where carbon monoxide exceeds established air-quality standards.

The USGS work is part of an interagency assessment of the scientific basis and effectiveness of the nation's oxygenated fuel program, which is coordinated by the President's Office of Science and Technology Policy. The request from the U.S. Environmental Protection Agency for this assessment was prompted by public complaints of headaches, nausea and other acute symptoms attributed to winter-time use of oxygenated gasoline, as well as complaints of reductions in gasoline economy and engine performance.

Oxygenates, such as MTBE, reduce the need for benzene and other ozone-forming, aromatic compounds in gasoline. MTBE, however, is less biodegradable than common gasoline compounds, such as benzene, toluene, ethylbenzene and total xylene (BTEX).

Concentrations of 62 volatile organic compounds (VOC's) and other constituents were measured in 592 stormwater samples collected in the 16 cities in 11 states, all of which have a population greater than 100,000. MTBE was the seventh most frequently detected VOC and was detected in 41 of the 592 stormwater samples collected. In decreasing order, the most frequently detected VOC's were toluene, total xylene, chloroform, trimethylbenzene, tetrachloroethene and naphthalene. VOC's are of concern because they are easily changed into gaseous form (volatilized) and, therefore, available to the air and water.

In each of the three cities -- Phoenix, Colorado Springs and Denver -- known to use MTBE, it was detected only in stormwater samples collected during the season when oxygenated gasoline was in use. MTBE was detected in 40 percent of the samples collected in these three cities during October through March.

Detection of MTBE in cities confirmed not to use oxygenated gasoline -- Atlanta, Baton Rouge, Birmingham, Dallas/Fort Worth and San Antonio -- may be attributable to the use of MTBE as an octane enhancer.

The cities sampled are required by the Clean Water Act to obtain permits for stormwater discharged from municipal separate stormwater systems into surface water. State air-pollution officials confirmed that MTBE was used in a winter oxygenated gasoline program during the sampling period in Colorado Springs, Denver and Phoenix. It was not used in the other 13 cities. In none of the cities was MTBE used in reformulated gasoline for carbon monoxide and ozone abatement. MTBE, however, may have been used in all or some of these cities to enhance the octane of gasoline.

In the study of ground water in New England, the network of wells sampled consisted of 103 monitoring wells in shallow aquifers, which are not used for drinking-water supply, and 30 domestic-supply wells in deeper bedrock aquifers. Seventy-seven percent of the MTBE detections were from samples collected from 26 shallow monitoring wells. MTBE was detected in 42 percent of the monitoring wells in urban areas. In agricultural areas, MTBE was detected in only 8 percent of monitoring wells and it was not detected in wells in undeveloped areas. Of the MTBE detections in shallow aquifers, 62 percent were from wells within 0.25 miles of gasoline stations or underground storage tanks. All but one of those wells were in Connecticut and Massachusetts where reformulated gasoline is used.

MTBE was detected in 23 percent of the deep domestic-supply wells that tap fractured bedrock aquifers. MTBE was detected in bedrock wells only in Connecticut and Massachusetts, where land use near the wells was suburban to rural. None of the sampled bedrock wells were within 0.25 miles of a gasoline stations. As with the urban stormwater study, all of the MTBE detections in ground-water sampled in New England were less than the lower limit of the USEPA's draft lifetime health advisory for drinking water.

The USGS work is a compilation of several USGS studies that were not specifically designed to assess gasoline oxygenates. The data were synthesized by the USGS National Water Quality Assessment Program, one goal of which is to provide a comprehensive assessment of the quality of most of the nation's water resources. MTBE is one of 60 VOC's being measured in ground-water and surface-water samples collected across the nation as part of the NAWQA program. Additional information on the occurrence of MTBE will be provided as the data become available.

Further information on the urban stormwater study is available in USGS Water-Resources Investigations Report 96-4145, "Occurrence of the Gasoline Oxygenate MTBE and BTEX Compounds in Urban Stormwater in the United States, 1991-95," by Gregory C. Delzer, John S. Zogorski, Thomas J. Lopes and Robin L. Bosshart. The report is available for $1.50 for paper copy or $4.00 for microfiche, plus $3.50 for shipping and handling, from the Branch of Information Services, USGS, Box 25286, Denver, Colo., 80225.

The report is also available via the Internet on the World Wide Web at: ml

The USGS has also published a fact sheet, "Environmental Behavior and Fate of Methyl tert-Butyl Ether (MTBE)," numbered FS 203-96, single copies of which are available free from the Denver address above. The fact sheet contains general and technical information on MTBE, its potential sources and how they can be determined, effects on surface and ground water and how MTBE behaves (for example, how quickly it travels in streams and rivers before it changes to a gas) and how easily MTBE can be removed from drinking water supplies. Recent research has demonstrated, for example, that bacterial populations and certain pure bacterial strains, when isolated from biotreated sludges and other sources, have the ability to use MTBE as a sole carbon source.

Further information on the ground-water study is available as USGS Open-File Report 95-456 on the USGS National Water Quality Assessment Program home page under the national synthesis study on VOC's. The address is:

General information on the USGS and links to the pages listed above are available at:

* * * USGS * * *

(Note to Editors: USGS spokesperson John Zogorski can be reached Friday only (Nov. 22) at the NAWQA program office (703) 648-5716. He will return to his home office Monday (Nov. 25) in Rapid City, S. Dak., at (605) 394-1780 ext. 214.)

US Geological Survey

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