National Water-Quality Assessment (NAWQA) Program

Design of the National Water-Quality
Assessment Program:

Occurrence and Distribution of Water-Quality Conditions

United States Geological Survey Circular 1112
By Robert J. Gilliom, William M. Alley, and Martin E. Gurtz


Many factors that affect the sources, behavior, and effects of contaminants and water-quality conditions are common to most hydrologic systems, although in widely varying degrees of importance. These common natural and human-related factors, such as geology and land use, provide a unifying framework for making comparative assessments of water quality within and among hydrologic systems at a wide range of scales and characteristics in different parts of the Nation. Characterizing this Environmental Framework is an essential element of the Occurrence and Distribution Assessment that cuts across all individual water-quality issues and components of Study-Unit and National-Synthesis studies. The Environmental Framework is used to compare and contrast findings on water quality within and among Study Units in relation to causative factors and, ultimately, to develop inferences about water quality in areas that have not been sampled.

Although it is beyond the scope of this report to fully describe plans for developing the information base for the Environmental Framework for each Study Unit, an overview is helpful to understand the approach to sampling design and data analysis for the Occurrence and Distribution Assessment. The Environmental Framework is developed at three general scales for each Study Unit:

Figure 5

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Figure 5. Conceptual stratification of Environmental Settings for the White River Basin Study Unit, Indiana.
Figure 6

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Figure 6. Environmental Settings of the White River Basin Study Unit, Indiana.

In the following description of the Occurrence and Distribution Assessment design, the implied development of extensive information on the Environmental Framework is a key aspect of the study design. Discussion of specific steps in the study-design process, such as choosing drainage basins with specific land-use features, gives the reader a general sense of the application of the Environmental Framework and the nature and scope of information requirements.

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