BACKGROUND In 1991, the U.S. Geological Survey (USGS) began a National Water-Quality Assessment (NAWQA) program. The long-term goals of the NAWQA program are to describe the status of, and trends in, the quality of a large, representative part of the Nation's surface- and ground-water resources and to identify the major natural and human factors that affect the quality of these resources. In addressing these goals, the program will produce a wealth of water-quality information that will be useful to policy makers and managers at the National, State, and local levels. The NAWQA program emphasis is on regional water-quality problems. The program will not diminish the need for smaller studies and monitoring designed and currently being conducted by Federal, State, and local agencies to meet their individual needs. The NAWQA program, however, will provide a large-scale framework for conducting many of these activities and an understanding about National and regional water-quality conditions that cannot be acquired from individual, small-scale programs and studies. Studies of 60 hydrologic systems that include parts of most major river basins and aquifer systems (study-unit investigations) are the building blocks of the National assessment. The 60 study units range in size from 1,000 mi 2 (square miles) to more than 60,000 mi 2 and represent 60 to 70 percent of the Nation's water use and population served by public water supplies. Twenty study-unit investigations were started in 1991, 20 additional are starting in 1994, and 20 more are planned to start in 1997. The Southern High Plains study unit was selected as one of 20 study units to begin assessment activities in 1994. This study will be run from the New Mexico District office of the USGS in Albuquerque, New Mexico.