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MTBE: Summary of Findings and Research by the U.S. Geological Survey

John S. Zogorski, Gregory C. Delzer, David A. Bender, Paul J. Squillace, Thomas J.
Lopes, Arthur L. Baehr, Paul E. Stackelberg, James E. Landmeyer, Carol J. Boughton,
Michael S. Lico, James F. Pankow, Richard L. Johnson, and Neil R. Thomson

U.S. Geological Survey, 1608 Mountain View Rd, Rapid City, SD 57702, USA
Phone (605) 355-4560 ext. 214, Telecopier (605) 355-4523,

The blending of methyl tert-butyl ether (MTBE) in gasoline in the United States has increased markedly since its first use in the late 1970's. Approximately 350 billion liters (2.2 billion barrels, 92 billion gallons) of MTBE were produced in 1997 and MTBE has become one of the largest-volume organic chemicals produced in the United States. The large-scale use of MTBE-containing gasoline inadvertently has resulted in its introduction to some surface and ground waters. MTBE is of concern to drinking-water utilities because of its low taste and odor threshold and possible human-health effects. The U.S. Environmental Protection Agency (USEPA) has issued a drinking-water advisory for MTBE of 20 to 40 micrograms per liter (mg/L). The release of MTBE-containing gasoline from storage tanks, pipelines, and other point sources has caused the concentration of MTBE in some ground water to exceed the USEPA advisory. Furthermore, such MTBE contamination has resulted in the abandonment of some domestic and community water supply wells, or necessitated the treatment of ground water prior to use for drinking purposes.

Through a combination of monitoring and research, the U.S. Geological Survey (USGS) is attempting to answer several important questions about MTBE contamination of water resources in the United States. The majority of USGS's studies to date have focused on determining the occurrence and distribution of MTBE in ambient ground water, stormwater, and most recently in small urban streams and in Lake Tahoe, California and Nevada. Research has also been initiated at a gasoline storage-tank release site to evaluate the potential for natural attenuation of ground water contaminated with MTBE and petroleum hydrocarbons. Furthermore, an assessment of the relative significance of the atmosphere and land surfaces as non-point sources of MTBE to water has begun. The primary questions that USGS scientists are attempting to answer include: (1) To what extent does MTBE occur in ground water, surface water, and drinking-water supplies? (2) Will MTBE move to deeper parts of aquifers and to drinking-water wells and, if so, what conditions will favor such transport? (3) What are the pathways by which non-point sources contribute MTBE to surface and ground water, and what is their relative importance? and (4) Will ground water contaminated with MTBE from gasoline spills be naturally attenuated and, if so, what are the important attenuation processes?

Much of USGS's MTBE monitoring and research is in a formative stage and few definitive conclusions have been reached at this time. However, several preliminary observations are noteworthy. First, ambient water-quality monitoring in the National Water-Quality Assessment (NAWQA) Program indicates frequent detection of MTBE in stormwater, small streams, and shallow ground water in those cities and metropolitan areas where MTBE is used extensively in gasoline. For example, of 60 volatile organic compounds (VOCs) for which water samples are commonly analyzed, MTBE is the second most frequently detected VOC in shallow ground water in urban areas, with a detection frequency of 27 percent. However, 97 percent of the detections are at concentrations below the USEPA drinking-water advisory. Second, because monitoring in the NAWQA Program is not targeted to point-source gasoline release sites or to effluent-dominated streams, most of the low-level detections of MTBE are thought to originate from non-point sources. Land surfaces and the atmosphere appear to be plausible non-point sources of MTBE to urban water resources. Admittedly, point sources located at some distance from sampled wells may also be a source. Third, MTBE has been detected at low levels in 14 percent of urban wells sampled from aquifers used for drinking water. In contrast, 2 percent of rural wells sampled from drinking-water aquifers contain MTBE. Lastly, concentrations of MTBE in ground water contaminated by a gasoline storage-tank release that occurred in the mid-1980's remain high and above the USEPA's drinking-water advisory. Furthermore, observed decreases in MTBE concentrations in ground water observed since 1993, when monitoring was begun at the site, are attributed primarily to abiotic processes of natural attenuation including dilution by mixing and some volatilization. Biodegradation is not a major attenuation process at this site.

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