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Results from two national surveys indicate that the gasoline oxygenate methyl tertiary butyl ether (MTBE) is one of the most frequently detected volatile organic compounds in source waters used by community water systems in the United States. Three other ether oxygenates were detected infrequently but almost always co‐occurred with MTBE. A random sampling of source waters across the United States found MTBE in almost 9% of samples. In geographic areas with high MTBE use, the compound was detected in 23% of source water samples. Although MTBE concentrations were low (<1 μg/L) in most samples, some concentrations equaled or exceeded the drinking water advisory of 20 μg/L set by the US Environmental Protection Agency. The frequent detection of even low concentrations of MTBE demonstrates the vulnerability of US source waters to anthropogenic compounds, indicating a need to include MTBE in monitoring programs to track the trend of contamination.

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Twenty-five volatile organic compounds (VOCs) were detected in water samples from 46 percent of wells in the Connecticut, Housatonic, and Thames River Basins NAWQA study area collected from July 1993 through September 1995. VOCs were detected in shallow monitoring wells (48 percent) screened in surficial aquifers and deeper, domestic-and institutional-supply wells (37 percent) completed in fractured bedrock. The gasoline additive MTBE was the most commonly detected VOC, followed by chloroform (25 and 23 percent of wells, respectively). VOC detections in ground water are related to urban land use, and the frequency of VOC detections and the total concentration of VOCs in ground water increased with increasing population density. Concentrations of five VOCs exceeded the U.S. Enivronmental Protection Agency's Maximum Contaminant Levels or Lifetime Health Advisories, but most (64 percent) VOC detections were at low concentrations (less than 1.0 microgram per liter) and may originate from nonpoint sources. NATIONAL WATER-QUALITY ASSESSMENT PROGRAM The National Water Quality Assessment (NAWQA) Program of the U.S. Geological Survey is designed to describe current waterquality conditions for a large part of the Nation's ground-and surface-water resources, to describe how water quality is changing over time, and to improve our understanding of the natural and human factors that afffect water quality. These goals are being achieved through investigations of 59 of the Nation's most important river basins and aquifer systems, referred to as study units. The Connecticut, Housatonic, and Thames River Basins study was among the first 20 investigations that began this water-quality assessment in 1991.

Introduction 2 Location and extent of the area 2 Purpose 2 Previous investigations 2 Methods of investigation 4 Acknowledgements 44

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The U.S. Geological Survey (USGS), in collaboration with the Metropolitan Water District of Southern California (MWDSC) and the Oregon Graduate Institute of Science and Technology (OGI) in Beaverton, Oregon, is designing a survey of the frequency of detection, concentration, and distribution of methyl te/t-butyl ether (MTBE),

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