For more information

Website:

A National Assessment of Volatile Organic Chemicals in Major Aquifer Systems and Rivers
http://wwwsd.cr.usgs.gov/nawqa/vocns/

Weighing One Environmental Concern Against Another:
The MTBE Story

In addressing one environmental concern and its potential health implications, others may arise. Such is the case with MTBE (methyl tert-butyl ether), the most commonly used fuel oxygenate.

Under the 1990 Clean Air Act Amendments, oxygenated fuel is required in areas -- usually urban -- where there are high levels of carbon monoxide or ozone. About 40 percent of the American population lives in these areas, and this percentage is expected to increase to 70 percent by 2000.

The upside to the story is that air quality has been significantly improved -- and carbon monoxide and ozone levels reduced -- where oxygenates have been added to gasoline. In areas where carbon monoxide or ozone concentrations have been reduced, however, MTBE has been found in the air, stormwater runoff, streams, lakes and reservoirs, and ground water. This unanticipated downside to the benefit of adding MTBE to fuels causes concern that water quality may degrade as air quality improves.

Volatile organic chemicals, such as MTBE, easily change into gaseous form (are volatized) and therefore are available to the air and water. MTBE does not biodegrade easily. It is very soluble and moves with water in the hydrologic cycle. Urban non-point sources, like precipitation and urban land-surface runoff, are thought to be important sources of the MTBE concentrations detected (mostly at low levels) in shallow ground water.

The USGS National Water Quality Assessment Program (NAWQA) has been studying the point (specific) and non-point (general) sources and distribution of MTBE and other synthetic volatile organic compounds (VOCs) in surface and ground water across the country. Analyses of more than 1,700 samples nationwide show that MTBE is detectable in surface- and ground-water resources in many areas. How frequently MTBE is detected in water and at what concentration directly relate to urban land use. Because MTBE use and its patterns and presence in atmospheric and hydrologic systems track so closely, the currently available data suggest that policies to control the effects of MTBE will benefit from a thorough understanding of how MTBE moves through the environment.

Pathways by which methyl tert-butyl ether moves through the environment.

Urban stormwater samples collected in 16 cities and metropolitan areas showed that although MTBE was detected, all detections were less than the lower limit of the U.S. Environmental Protection Agency's (EPA) 1997 Drinking Water Advisory of 20 to 40 parts per billion.

The use of recreational boats on lakes and reservoirs is another source of MTBE in the water environment. Such use has been found to produce concentrations of MTBE up to 10 parts per billion. Where much larger concentrations of MTBE are found in water, they result from point-source releases, such as leaking underground storage tanks, large spills, and wastewater discharges. In rare instances, MTBE concentrations can be very high, but such high concentrations are easily traced to large spills or leaking storage tanks.

MTBE in water samples is generally detected most frequently and at the highest concentrations in urban areas. Even in shallow urban ground water, however, concentrations are usually low in comparison to the EPA Drinking Water Advisory. The advisory levels primarily reflect taste and odor considerations. Although MTBE has been tentatively classified as a possible human carcinogen, EPA believes that, based on current knowledge, the advisory level provides a wide margin of public health safety. Studies continue to further define the health risks.

USGS studies of MTBE and other VOCs will give policy makers, resource managers, and environmental health officials the scientific information they need to weigh the trade offs between air-quality improvements and potential adverse water-quality impacts.

MTBE Detected in New England Ground Water

The USGS National Water Quality Assessment Program recently reported results of a ground-water resources study in an area of high MTBE use in central and southern New England in 133 wells sampled in the Housatonic, Connecticut, and Thames River basins.

MTBE was the most commonly detected VOC. It was found in 25 percent of the wells. Because of the strong correlation with land use, none of the samples from wells in forested areas had MTBE, but 40 percent of wells in urban areas had MTBE at detectable levels.

Other NAWQA studies have indicated that MTBE moves with infiltrating water that recharges the surficial aquifer through unsaturated soils and bedrock fractures. These New England detections may result from surface waters, with MTBE entering the wells through leaky well seals or shallow fractures rather than being an indication of contamination deep within the aquifer.

EPA and USGS Study Drinking Water in 12 Northeastern States

The EPA and USGS are studying MTBE and other VOC contaminants in the drinking water supplies of 12 Northeastern States from Virginia north through New England. The 65 million people who live in this region use large amounts of oxygenated gasoline, which contains MTBE. The cooperative study will use data collected since 1993 by 2,100 of the 10,500 community water systems. Data from other Federal and State programs will also be used.

EPA will gain important information on potential human exposure from drinking water to MTBE and other unregulated and regulated VOCs. The USGS will gain an increased understanding of the factors that affect the distribution of MTBE and other VOCs in a large, highly urbanized part of the Nation. Study results will be used by water resource managers in areas with similar land use, and geologic and hydrologic characteristics.

MTBE Frequently Detected in New Jersey

New Jersey is densely populated and highly urbanized, which makes it not too surprising that MTBE and other VOCs are found in surface- and ground-water resources throughout the State. USGS studies in New Jersey are investigating the occurrence and distribution of MTBE and other VOCs in relation to land-use patterns, geohydrology, soils, and other natural and human factors. More specifically, they are conducting a broad assessment of ground-water quality conditions in aquifers used for public supply.

Sampling of surface waters over the past few years has shown that MTBE is one of the most frequently detected VOCs. Frequency and concentrations are correlated with urbanization and population density, as is the case elsewhere in the country.

In the rapidly growing Glassboro area, the surficial Kirkwood-Cohansey aquifer system is becoming an important source of public water supply. This underground water supply is recharged directly from surface water that infiltrates the aquifer through overlying unconsolidated sands and gravels, and is, therefore, vulnerable to contamination from that surface-water influence. The USGS has established a network of 72 wells to monitor recently recharged ground water. In the initial sampling, MTBE was detected in 44 percent of the wells.