complacent about water resources because of changes in population and consumption. Challenges to be met are numerous and include issues of water availability and accessibility, water quality, and hydrologic hazards. These issues are not new to the USGS. For example, the USGS has collected and analyzed data on water quality for more than 100 years (NRC, 1990).

Nations vulnerable to water scarcity are primarily in the arid or semiarid regions of Africa, Asia, and the Middle East. Altogether about 232 million people in 26 countries are living in regions considered “water scarce” (NRC, 1999b). Yet water availability is also an issue in some regions of the United States, which is a water-rich nation. In recent decades, the arid Southwest and West have begun to face the limits of water availability as a result of burgeoning population and development. Continued growth and development will require some combination of importing water and using and managing it more efficiently. They will also require that a balance be struck between competing uses. As in the case of other water-scarce regions of the world, conflicts among water users (e.g., agriculture, industry, households) and between ecosystems and regions (e.g., uplands, floodplains, cities) may become an increasing problem. In parts of the world, the scarcity of water also could be a source of conflict between nations (e.g., South Asia, Middle East).

Water quality is declining in developing countries, especially in urban areas. Although the degradation of water quality can be arrested and sometimes reversed, the process is slow and costly, as exemplified by the experience of developed countries over the past 30 years. In the United States, passage of the Clean Water Act in 1972 resulted in marked improvement in water quality of streams and rivers that receive discharges from municipal waste treatment plants and industrial facilities (point source pollution). Further efforts to improve the quality of the nation's water will require a reduction of pollution from diffuse (nonpoint) sources that include storm water runoff and runoff from agricultural fields and livestock wastes (NRC, 2000a) (Sidebar 3.1). In most cases, nonpoint sources of pollution are difficult to treat and identify.

Hazardous materials in the hydrologic environment are a problem of substantial national significance (NRC, 1996b) and call for the expertise of scientists. The role of the USGS in this arena is to expand scientific knowledge relevant to the behavior of hazardous materials. The generation and storage of toxic chemical and radioactive wastes will be of increasing