USGS

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

GOALS AND MAJOR DESIGN COMPONENTS
OF THE OCCURRENCE AND DISTRIBUTION
OF WATER-QUALITY CONDITIONS

The Occurrence and Distribution Assessment is designed to meet goals at Study-Unit and national scales for surface- and ground-water issues (table 1). These goals are met to varying degrees in different Study Units and for different regional and national water-quality issues. The degree of success depends on such factors as availability of existing data, Study-Unit size and complexity, and the nature and complexity of the factors that govern particular water-quality conditions. In addressing the goals, NAWQA staff at all levels work closely with local, State, and Federal agencies, and the public through the liaison and advisory committees to assess potential implications of study results for management priorities and information requirements in the Study Units and nationally.

Figure 4 shows the study components of the Occurrence and Distribution Assessment. This report is organized by this design hierarchy and explains the role and design of each component.

Table 1. General goals of the Occurrence and Distribution Assessment
I. OCCURRENCE
A. Study Unit -
Identify the occurrence of water-quality conditions that are significant issues.
B. National Synthesis -
Identify the occurrence of water-quality conditions that are regionally and nationally significant water-quality issues.
II. SPATIAL AND TEMPORAL DISTRIBUTION
A. Study Unit -
Characterize the broad-scale geographic and seasonal distribution of a wide range of water-quality conditions in relation to natural factors and human activities.
B. National Synthesis -
Characterize and compare the geographic and seasonal distribution of selected water-qualtiy conditions among the broad range of natural and land-use settings of the Nation.
III. DESIGN OF STUDIES FOR ASSESSING LONG-TERM TRENDS AND CHANGES
A. Study Unit -
Evaluate Study-Unit priorities and required study approaches for effectively assessing long-term trends and changes.
B. National Synthesis -
Work with Study Units to develop nationally consistent approaches to assess long-term trends and changes for priority issues.
IV. PRELIMINARY ASSESSMENT OF SOURCES, TRANSPORT, FATE, AND EFFECTS

A. Study Unit -
For water-quality conditions of greatest importance, evaluate geographic and seasonal distributions in greater detail and in relation to the sources, transport, fate, and effects of contaminants. Determine the priorities and design for followup case studies.
B. National Synthesis -
Characterize and compare the similarities and differences in relations to sources, transport, fate, and effects of selected contaminants prioritized for National Synthesis across the range of natural and anthropogenic settings of the Nation.

Figure 4.

(Click on image for a larger version, 52K GIF)

Figure 4. Components of the Occurrence and Distribution Assessment.


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